The `FILE_PT_operator` panel used in the file browser does not correctly
allow the new UI layout panels to be drawn correctly.
The layout panels depend on drawing the background so that different
colors can denote each panel section. In the case of `FILE_PT_operator`,
this background drawing is skipped entirely leading to a situation where
there's no panel delineation at all.
Forcing the background to be drawn leads to a second problem where
the "typical" panel colors are not used in this part of the file
browser. We need to match the surrounding area otherwise a much
lighter shade of gray will be used and look out of place.
The fix is to extend the processing for PANEL_TYPE_NO_HEADER to account
for both of the above situations.
Pull Request: https://projects.blender.org/blender/blender/pulls/118231
c8f2a5cc42