Blender Git Loki

Old feature request: undo system now restores images without reloading.
Was surprisingly easy to add, compiant with file reading and undo code.

Currently only the Image->ibuf gets restored, and its opengl binding, so
for realtime texture it works nicely. Also texture images are not freed
inbetween undo steps

Notes:
- Painting textures will just keep the painted image, there's no undo
yet for that
- If this works satisfying, I'll extend it to compositing previews

TEST IT WELL PLEASE! :)