Blender Git Loki

Fix #32951: Progressive refine crashing Blender when used with multiple cuda devices

This time issue was caused by Save Buffers enabled and the reason blender
crashed was tiles generated by cycles didn't match tiles generated by
openexr.

Currently solved it in a way that background rendering will allocate tiles
in the same way render parts are generated in blender. Viewport rendering
will still be using sliced image, but it's more tricky to change and it's
no so big deal for now.