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! :)
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