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Texture Paint: changing paint slots and viewport could go out of sync

When changing to another texture paint slot, the texture displayed in
the viewport should change accordingly (as well as the image displayed
in the Image Editor).

The procedure to find the texture to display in the viewport
(BKE_texpaint_slot_material_find_node) could fail
though because it assumed iterating nodes would always happen in the
same order (it was index based). This is not the case though, nodes can
get sorted differently based on selection (see ED_node_sort).

Now check the actual image being referenced in the paint slot for
comparison.

ref T88788 (probably enough to call this a fix, the other issue(s)
mentioned in the report are more likely a feature request)

Reviewed By: mano-wii

Maniphest Tasks: T88788

Differential Revision: https://developer.blender.org/D11496