Auto-save currently only really works in modes that use the `MemFile` undo step,
that excludes things like mesh edit and sculpt mode. Previously, Blender would
attempt to auto-save in those modes, but it would only save the last state from
before the mode was entered, which is useless when staying in the mode for longer.
This problem is *not* fixed here. However, the code now explicitly skips auto-saving
in order to avoid unnecessary short freezes in these modes when Blender auto-saves.
Furthermore, the auto-save will now happen when changing modes.
This reduces the impact of save-time-regressions with #106903.
Pull Request: https://projects.blender.org/blender/blender/pulls/118892
41b10424c7