This resolves a bottleneck where every update while transforming
copied the entire mesh data-block, which isn't needed as the edit-mesh
is the source of the data being edited.
Testing shows a significant overall speedup when transforming:
- ~1.5x with a subdivided cube 1.5 million vertices.
- ~3.0x with the spring mesh (edit-mode with modifiers disabled,
duplicated 10x to drop performance).
Reviewed By: sergey
Ref D11337
51568030e9