fix: 解决canvas组件绘制后某些情况被清空的问题

build uni-h5 0.0.9

packages/uni-h5/package.json

 {
 	"name": "@dcloudio/uni-h5",
-	"version": "0.0.8",
+	"version": "0.0.9",
 	"description": "uni-app h5",
 	"main": "dist/index.umd.min.js",
 	"scripts": {

src/core/view/components/canvas/index.vue

@@ -68,8 +68,11 @@ export default {
       }
     },
     _resize ({ width, height }) {
-      this.$refs.canvas.width = width
-      this.$refs.canvas.height = height
+      var canvas = this.$refs.canvas
+      if (canvas.width !== width || canvas.height !== height) {
+        canvas.width = width
+        canvas.height = height
+      }
     },
     _touchmove (event) {
       if (this.disableScroll) {