fixed memory leak for non flash targets

It is not a real leak, but on non flash targets the canvas buffer allocates more memory then needed.

The buffer on [Canvas.hx l.57](https://github.com/HaxePunk/HaxePunk/blob/e992a0ee3540ffe70e91cc3c0a08728da778bfda/com/haxepunk/graphics/Canvas.hx#L57) is used for rendering Canvas and Tilemaps at flash target only but is filled on every target what, at least, is a problem on mobile devices. 

This patch has dropped the memory from ~700mb to 21mb on our project.