- Store imbuf file path in UndoImageTile structure, so imbuf can be verified
before applying titles on it.
- If current image's imbuf file path isn't equal to file path stored in
undo block, search for imbuf in ima->ibufs.
Probably it can be optimized, but storing all settings which defines
needed imbuf (image source type, offset, image file name, current
scene frame and so messes up undo code and requires deeper changes
which probably better not to start on bcon4).
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