Support for having an animationsheet inside a packed texture

#373 

Dont think this is quite ready for merging in, but I wanted to start a discussion.

I know we advise people to not use rotation when it comes to texture packer, but I feel this should support it. However, I wasnt sure on how best to implement. Will setting the source angle work? My concern is with how the math is calculated on generating the frames :). Maybe it's just a matter of taking the angle and flipping the sprite width & height values, along with the indices?