Blender Git Loki

Undo System: avoid redundant decoding on undo

In most cases the undo system was loading undo steps twice.

This was needed since some undo systems (sculpt, paint, text)
require stepping out of the current undo step.

Use a flag to limit this to the undo systems that need it.

This improves performance for other undo systems.
This gives around 1.96x speedup in edit-mesh for high-poly objects.

Reviewed by: mont29

Ref D11893