Apply blend modes on layer level and add Multiply blend mode (#2519)

This commit improves blend mode support in lottie-android in two ways:
* Applying blend modes on layer-level, instead of fill level
* Adding support for the Multiply blend mode

## Applying blend modes on layer level

The Lottie format defines blend modes as attributes on a layer. However, lottie-android is presently applying the layer blend modes on a solid color fill only. Notably, this causes any stroked or gradient-filled shapes or image layers to blend incorrectly, such as in this file:

[stroke-blending-test.json](https://github.com/user-attachments/files/16346206/stroke-blending-test.json)

(The file contains a filled + stroked shape that renders as a pink square on other platforms, but renders with a visible stroke on lottie-android since its blend mode is applied only on the fill.)

Instead, we move this decision to `BaseLayer` by analogy to transparent layer handling, which is closer to how the format specifies the property and fixes these cases.

## Multiply support

`BlendModeCompat` is designed to resolve to either a `BlendMode` (added in Android Q, supporting most modern blend modes) or `PorterDuff.Mode` (always available, but a smaller choice of modes as it is mostly focused on alpha compositing).

We use `BlendModeCompat` to support Lottie layer blend modes (`bm` key) to ensure compatibility on all platforms. For consistency, we don't  support values which don't have a `PorterDuff.Mode` equivalent.

Our support for Lottie blend modes did not include Multiply due to a slightly different behavior between the `PorterDuff.MULTIPLY` (exposed as `BlendModeCompat.MODULATE`) and `BlendModeCompat.MULTIPLY` variants. Namely, the formula used for `PorterDuff.MODULATE`, combined with alpha-premultiplication done by Skia, means that a layer with an alpha < 1.0 and multiply blend mode will also darken the destination:

![Incorrect-Blend](https://github.com/user-attachments/assets/6a2113ef-4bac-4bbc-830b-1353adf4ee2b)

(Multiply-blended layers with < 1.0 alpha on the left, Screen-blended layers with < 1.0 alpha on the right)

However, what we can do instead is clear the canvas with a solid white color instead of transparent before drawing the layer's contents as normal. When blending the resulting bitmap over an opaque background using `PorterDuff.MULTIPLY` (i.e. `BlendModeCompat.MODULATE`), the end result will be as if we had used `BlendModeCompat.MULTIPLY`, since all-1.0 (white) is a multiplication identity:

![Correct-Blend](https://github.com/user-attachments/assets/126022ef-6e47-48ee-b803-1d9800ca2c75)

This PR implements the latter solution and adds a consistent support for the Multiply blend mode for all Android versions.

*Test file used:*  [blendmode-tests-multiply+screen+bg.zip](https://github.com/user-attachments/files/16365843/blendmode-tests-multiply%2Bscreen%2Bbg.zip)

Author: geomaster

lottie/src/main/java/com/airbnb/lottie/animation/content/FillContent.java

@@ -11,7 +11,6 @@
 import android.graphics.RectF;
 
 import androidx.annotation.Nullable;
-import androidx.core.graphics.PaintCompat;
 
 import com.airbnb.lottie.L;
 import com.airbnb.lottie.LottieDrawable;
@@ -68,8 +67,6 @@ public FillContent(final LottieDrawable lottieDrawable, BaseLayer layer, ShapeFi
       return;
     }
 
-    PaintCompat.setBlendMode(paint, layer.getBlendMode().toNativeBlendMode());
-
     path.setFillType(fill.getFillType());
 
     colorAnimation = fill.getColor().createAnimation();

lottie/src/main/java/com/airbnb/lottie/model/content/LBlendMode.java

@@ -33,6 +33,20 @@ public BlendModeCompat toNativeBlendMode() {
     switch (this) {
       case NORMAL:
         return null;
+      case MULTIPLY:
+        // BlendModeCompat.MULTIPLY does not exist on Android < Q. Instead, there's
+        // BlendModeCompat.MODULATE, which maps to PorterDuff.Mode.MODULATE and not
+        // PorterDuff.Mode.MULTIPLY.
+        //
+        // MODULATE differs from MULTIPLY in that it doesn't perform
+        // any alpha blending. It just does a component-wise multiplication
+        // of the colors.
+        //
+        // For proper results on all platforms, we will map the MULTIPLY
+        // blend mode to MODULATE, and then do a slight adjustment to
+        // how we render such layers to still achieve the correct result.
+        // See BaseLayer.draw().
+        return BlendModeCompat.MODULATE;
       case SCREEN:
         return BlendModeCompat.SCREEN;
       case OVERLAY:
@@ -48,7 +62,6 @@ public BlendModeCompat toNativeBlendMode() {
       // To prevent unexpected issues where animations look correct
       // during development but silently break for users with older devices
       // we won't support any of these until Q is widely used.
-      case MULTIPLY:
       case COLOR_DODGE:
       case COLOR_BURN:
       case HARD_LIGHT:

lottie/src/main/java/com/airbnb/lottie/model/layer/BaseLayer.java

@@ -14,6 +14,7 @@
 import androidx.annotation.CallSuper;
 import androidx.annotation.FloatRange;
 import androidx.annotation.Nullable;
+import androidx.core.graphics.PaintCompat;
 
 import com.airbnb.lottie.L;
 import com.airbnb.lottie.LottieComposition;
@@ -117,6 +118,8 @@ static BaseLayer forModel(
   float blurMaskFilterRadius = 0f;
   @Nullable BlurMaskFilter blurMaskFilter;
 
+  @Nullable LPaint solidWhitePaint;
+
   BaseLayer(LottieDrawable lottieDrawable, Layer layerModel) {
     this.lottieDrawable = lottieDrawable;
     this.layerModel = layerModel;
@@ -258,7 +261,7 @@ public void draw(Canvas canvas, Matrix parentMatrix, int parentAlpha) {
       }
     }
     int alpha = (int) ((parentAlpha / 255f * (float) opacity / 100f) * 255);
-    if (!hasMatteOnThisLayer() && !hasMasksOnThisLayer()) {
+    if (!hasMatteOnThisLayer() && !hasMasksOnThisLayer() && getBlendMode() == LBlendMode.NORMAL) {
       matrix.preConcat(transform.getMatrix());
       if (L.isTraceEnabled()) {
         L.beginSection("Layer#drawLayer");
@@ -307,13 +310,31 @@ public void draw(Canvas canvas, Matrix parentMatrix, int parentAlpha) {
         L.beginSection("Layer#saveLayer");
       }
       contentPaint.setAlpha(255);
+      PaintCompat.setBlendMode(contentPaint, getBlendMode().toNativeBlendMode());
       Utils.saveLayerCompat(canvas, rect, contentPaint);
       if (L.isTraceEnabled()) {
         L.endSection("Layer#saveLayer");
       }
 
       // Clear the off screen buffer. This is necessary for some phones.
-      clearCanvas(canvas);
+      if (getBlendMode() != LBlendMode.MULTIPLY) {
+        clearCanvas(canvas);
+      } else {
+        // Due to the difference between PorterDuffMode.MULTIPLY (which we use for compatibility
+        // with Android < Q) and BlendMode.MULTIPLY (which is the correct, alpha-blended mode),
+        // we will alpha-blend the contents of this layer on top of a white background before
+        // we multiply it with the opaque substrate below (with canvas.restore()).
+        //
+        // Since white is the identity color for multiplication, this will behave as if we
+        // had correctly performed an alpha-blended multiply (such as BlendMode.MULTIPLY), but
+        // will work pre-Q as well.
+        if (solidWhitePaint == null) {
+          solidWhitePaint = new LPaint();
+          solidWhitePaint.setColor(0xffffffff);
+        }
+        canvas.drawRect(rect.left - 1, rect.top - 1, rect.right + 1, rect.bottom + 1, solidWhitePaint);
+      }
+
       if (L.isTraceEnabled()) {
         L.beginSection("Layer#drawLayer");
       }