Caused by rBebaa3fcedd23: Fix T84398: Multiview images show only one view..

I dont quite follow above commit, seems this assumed an ImageUser's
multi_index is only used for Multiview/Stereo? This is not the case,
multi_index also stores the index for layer/pass combination.

If we call both BKE_image_multilayer_index and BKE_image_multiview_index
(even though this is not appropriate/needed for multilayer images?), we
might end up overwriting multi_index again.

There might be a better solution for this (as I said, I might have
gotten the intention behind rBebaa3fcedd23 wrong).

note: looking at this I was also wondering why we update the ImageUser
in image-buffer-aquiring funnctions [and not from the UI, e.g.
template_image_layers, but that is a whole different story I guess, see
comment in T90772 as well]