Blender Git Loki

Fix T77456: Broken vertex paint undo on high-poly objects.

This is a critical fix that should also be backported to 2.83.1

Fairly stupid bug in fact, code detecting changes across undo steps was
assuming that each BHEAD (a block of data in blendfiles) would not be larger
than one memory chunk... Which is the case in alsmost every situation,
besides some super-heavy geometries, and other similar things (images
would also be affected e.g.).