Blender Git Loki

Fix T75920: Add object - Align to 3D cursor not working.

3DCursor is UI data (hence not expected to be affected by undo) that is
stored in actual data (Scene)... So it needs some special care during
undo.

New undo code now re-reads data into existing memory, which means
copying of 3DCursor data has to happen earlier in that case, when we
still have both old and newly read data available.