A regression since d579ac2b3f.
Cycles delays allocation of the render buffers, which makes it impossible
to access effective render resolution during rendering. This was making the
drawing code to fall-back to the full scene resolution with only percentage
and crop accounted for.
This change makes it so an empty ImBuf is used to communicate the render
result resolution. It does not have any pixel buffer associated with it,
which actually matches the behaivor prior to the offending commit.
Pull Request: https://projects.blender.org/blender/blender/pulls/113282
ba767610cf