Fixed clipping issue for PIXI v5.

lib/pixi/deepzoom/image.js

@@ -386,11 +386,10 @@ export class DeepZoomImage extends PIXI.Container {
 
         if (deepZoomInfo.clip) {
             let mask = new PIXI.Graphics()
-            mask.beginFill(1, 1)
+            mask.beginFill(0xff0000, 1)
             mask.drawRect(0, 0, w, h)
             mask.endFill()
             this.mask = mask
-            mask.alpha = 0
             this.addChild(mask)
             this.minimumLevel = deepZoomInfo.baseLevel
         }