Handling premultiplication issues in a round-about way.

You can render the packed images with or without premultiplication.
SpriteSheet will premultiply non-premultiplied sheets at runtime.
This helps deal with XCode's auto-premultiplication step for iOS if
you want to share the same graphics across platforms.
You can also just generate premultiplied and non-premultiplied versions
depending on your platform/tool needs.