Bitmap Mask updates
Bitmap Mask updates

* The `WebGLRenderer` has 4 new properties: `maskTargetFramebuffer`, `maskSourceFramebuffer`, `maskTargetTexture` and `maskSourceTexture`. These are the new global locations of the mask framebuffers.
* `WebGLRenderer.createBitmapMask` is a new method that internally creates the Bitmap Mask framebuffers.
* `WebGLRenderer.clearBitmapMask` is a new method that internally clears the existing Bitmap Mask framebuffers, called as part of a resize event.
* `WebGLRenderer.enableBitmapMask` is a new method that starts the process of using the mask target framebuffer for drawing. This is called by the `BitmapMaskPipeline`.
* `WebGLRenderer.drawBitmapMask` is a new method that completes the process of rendering using the mask target framebuffer. This is called by the `BitmapMaskPipeline`.

photonstorm/phaser,photonstorm/phaser