Added alpha setting to getContext.

Author: photonstorm

src/gameobjects/BitmapData.js

@@ -51,7 +51,7 @@ Phaser.BitmapData = function (game, key, width, height) {
     * @property {CanvasRenderingContext2D} context - The 2d context of the canvas.
     * @default
     */
-    this.context = this.canvas.getContext('2d');
+    this.context = this.canvas.getContext('2d', { alpha: true });
 
     /**
     * @property {CanvasRenderingContext2D} ctx - A reference to BitmapData.context.