Update image undo to store buffers for each step:
- Undo buffers share tiles to avoid using too much memory.
- Undo support for different sized buffers
allowing operations such as crop or resize.
- Paint tiles have been split into separate API/storage.
- Painting speed wont be impacted significantly
since storing the extra tiles is done after the stroke & only
for the first undo step.
Resolves T61263, see D5939 for details.
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