fix(create-canvas-context-async): 高清逻辑太晚导致宽高不对

pages/API/create-canvas-context-async/create-canvas-context-async.uvue

@@ -46,11 +46,10 @@
         component: this,
         success: (context : CanvasContext) => {
           this.canvasContext = context.getContext('2d')!;
+          hidpi(this.canvasContext!.canvas);
           this.canvasWidth = this.canvasContext!.canvas.width;
           this.canvasHeight = this.canvasContext!.canvas.height;
 
-          hidpi(this.canvasContext!.canvas);
-
           // #ifdef WEB
           context.toBlob((blob : Blob) => {
             this.testToBlobResult = (blob.size > 0 && blob.type == 'image/jpeg')