Caused by own rB5aa3167e48b2: Fix T90772: Image Editor not sampling color from the the currently.
Related commit: rBebaa3fcedd23: Fix T84398: Multiview images show only one view..

For stereo renders, BKE_image_is_multilayer is true, however we seem
to get to down to space_image_gpu_texture_get [where this is called]
from IMAGE_cache_init with a NULL Image->RenderResult.

So what then happens is that BKE_image_multilayer_index is called and
even though it has an appropriate codepath for stereo, it earlies out
and does not set multi_index correctly.

Still a bit puzzled why RenderResult is NULL for a render, but since
other places also check for a valid RenderResult before going down the
_multilayer_ route (and doing _multiview_ instead), now do the same
thing, BKE_image_multiview_index is now called in these cases (and seems
to behave correctly, checked with layers and passes and all seems to
display correctly, either in stereo or choosing individual eyes).

Since this is somewhat a stab in the dark regarding the NULL
RenderResult, I would appreciate an extra careful eye.