browser-canvas: 调整 UniElement 为 HTMLElement

pages/template/browser-canvas/browser-canvas.uvue

@@ -13,13 +13,13 @@
     data() {
       return {
         title: '浏览器 canvas',
-        drawing: null as UniElement | null,
+        drawing: null as HTMLElement | null,
         canvasElement: null as HTMLCanvasElement | null,
         canvasContext: null as CanvasRenderingContext2D | null,
       }
     },
     onReady() {
-      this.drawing = this.$refs['drawing'] as UniElement;
+      this.drawing = this.$refs['drawing'] as HTMLElement;
       this.canvasElement = document.createElement('canvas') as HTMLCanvasElement;
       this.canvasElement.className = 'canvas'
       this.drawing!.appendChild(this.canvasElement)