Allow material to control offset and repeat.

I'm making a 2D game which uses lots of sprites. I animate the sprites by switching between subregions of the texture with the "offset" and "repeat" parameters. Each sprite needs to be animated independently. However, I don't want to clone the whole texture for each instance of a sprite.

This change allows the user to assign "offset" and "repeat" values to a Material (where they really belong, I think) instead of a Texture.