Turn off pbvh normal update flag after recalculation, saves
recalculating normals every frame when not stroking the mesh.
For this to work reliably with undo we need to support original normals
in the bm_log (was marked as a TODO already in the code), so that
undoing avoids having invalid normals in the mesh (since we don't update
every frame anymore). This was added in this commit as well.
Also added some (disabled) quite paranoid checks in the bmesh valication
code for dyntopo hoping to catch the real normal update issue. No luck
there yet.
6f88dca9c3