entered with Plater::enter_gizmos_stack(), and left with
Plater::enter_gizmos_stack(). Other than that, the 2nd Undo / Redo
stack is transparent to the user of the Plater.
WIP: Currently the Gizmo stack takes a snapshot of the whole scene on
Plater::enter_gizmos_stack(). While it should work, it may be cheaper
to modify the Undo/Redo stack to only take a snapshot of the gizmos
in the Gizmo mode.
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