Blender Git Loki

Fix assert for Image Editor invert/resize operators

Caused by rB2bf4c74130ff.

For undo, the ImageUser's scene should be NULL (see D7022 for
discussion).

PaintTiles were already doing it beforehand in ED_image_paint_tile_push,
but Image ops [scale/invert] are calling
ED_image_undo_push_begin_with_image directly.

Now actually set the UndoImageHandle iuser.scene to NULL (rather than
asserting)

ref T75675

Maniphest Tasks: T75675

Differential Revision: https://developer.blender.org/D7435