Blender Git Loki

Fix T75675: Unlinking [with setting users to zero] not clearing
LIB_TAG_EXTRAUSER_SET flag

For example in the Image Editor, an assert would be triggered after
unlinking an image [with setting users to zero] and then setting the
image for the Image Editor again.

Whenever we set an Image for Image Editor, the Image ID is flagged
LIB_TAG_EXTRAUSER_SET, when we unlink [with setting users to zero] this
flag was not cleared.

quote @mont29: "a proper fix would be to move this to modern code, and
actually delete the ID..." but that is for later.

Maniphest Tasks: T75675

Differential Revision: https://developer.blender.org/D7452