Blender Git Loki

Optimize sculpt undo, avoid redundant updates

On undo, sculpting regular meshes would update _all_ GPU VBO's.
Avoiding the update gives noticeably faster undo.

This is also a fix/workaround for strange behavior with NVidia's driver (T47232),
Where locking and unlocking all buffers for updating
slows down redraw speed permanently after the first undo.

However the problem isn't avoided entirely since a single brush stroke might modify most of the mesh.