Fixing the Graphics class

1. Removed bounds redundancy. this.bounds was removed and replaced with the inherited this._bounds.
2. Added cachedSprite support to Canvas graphics.
3. Added rounded rect to updateBounds.
4. Pushing the updated cached sprite texture to the gpu now.
5. bounds is no longer overwritten with new PIXI.Rectangle classes, instead only the values x,y,width and height get updated.

Fixes the bug that generateTexture is empty. this.bounds was always defined by an empty PIXI.Rectangle. The condition if(!this.bounds)this.updateBounds(); was always false.

Please review my changes. Hopefully no bugs in it.