fix: use var

Author: weizhenye

README.md

@@ -2,7 +2,6 @@
 
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package.json

@@ -43,7 +43,7 @@
   "homepage": "https://danmaku.js.org/",
   "devDependencies": {
     "@rollup/plugin-replace": "^2.3.3",
-    "chai": "^4.2.0",
+    "chai": "4.2.0",
     "es6-promise": "^4.2.8",
     "eslint": "^7.6.0",
     "karma": "^5.1.1",

src/engine/canvas.js

@@ -1,4 +1,4 @@
-const dpr = typeof window !== 'undefined' && window.devicePixelRatio || 1;
+var dpr = typeof window !== 'undefined' && window.devicePixelRatio || 1;
 
 export var canvasHeightCache = Object.create(null);