If render engines dont specify `RE_USE_PREVIEW` / `bl_use_preview`, they
wont pass the `ED_check_engine_supports_preview` test.
That is mostly meant to tell blender this engine is capable of
generating (Material) previews and if it isnt, they will skip generating
button previews in `icon_preview_startjob_all_sizes`.
There are already exceptions for type of IDs that can still generate
previews (think Asset previews for Objects or Actions), so the same
exception can be made for Images and Brushes as well (these previews
will just befulled from the Image buffer, that should not rely on the engine
whatsoever). We can also be permissive on Groups (they are treated
similar to Objects).
NOTE: this was reported for the Workbench engine, in another (future)
commit, we might even consider flagging that engine `RE_USE_PREVIEW` as
well (see comments in the PR)?
Pull Request: https://projects.blender.org/blender/blender/pulls/136824
f13bf270c6