Change DisplayObject generateTexture method signature to match Pixijs signature

generateTexture method now matches:
http://www.goodboydigital.com/pixijs/docs/classes/DisplayObject.html#method_generateTexture

`pixi.primitives.Graphics` class has overloaded the generateTexture method to use only two parameters. So at the moment you have to pass `null` in as the last parameter.
Any thoughts on this?