Clear the canvas in CanvasView before drawing

If the CanvasView doesn't clear the canvas before copying from
the master canvas, transpartent parts originating from clear_rect
will not appear in the CanvasViews.

Author: JeremiasE

src/widget.ts

@@ -328,6 +328,7 @@ class CanvasView extends DOMWidgetView {
   }
 
   updateCanvas() {
+    this.clear();	
     this.ctx.drawImage(this.model.canvas, 0, 0);
   }