Blender Git Loki

UndoSys: Refactor step 1: cleanup and simplify logic in main undo/redo handlers.

* Simplify and clarify logic in `BKE_undosys_step_undo/redo_with_data_ex`,
by adding early return on invalid situations, renaming some variables,
and adding comments.
* Add more sanity checks in those functions.

No behavioral change are expected here, besides in potential edge-case,
invalid situations.

This is a preliminary change, before some deeper modifications of
`BKE_undosys` undo/redo API.

Differential Revision: https://developer.blender.org/D10033