Increase epsilon for bilinear patch self-intersection test.

Work-around for infinite loop in subsurface scattering in #416.

Author: mmp

src/pbrt/shapes.h

@@ -1290,7 +1290,7 @@ PBRT_CPU_GPU inline pstd::optional<BilinearIntersection> IntersectBilinearPatch(
 
     // Find epsilon _eps_ to ensure that candidate $t$ is greater than zero
     Float eps =
-        gamma(10) * (MaxComponentValue(Abs(ray.o)) + MaxComponentValue(Abs(ray.d)) +
+        gamma(30) * (MaxComponentValue(Abs(ray.o)) + MaxComponentValue(Abs(ray.d)) +
                      MaxComponentValue(Abs(p00)) + MaxComponentValue(Abs(p10)) +
                      MaxComponentValue(Abs(p01)) + MaxComponentValue(Abs(p11)));