Remove all the roadblocks we've put up to keep implicit modifiers
out. Our importing code already handles them as a signal that says
'No modifiers, please!'. Now we just hope for the best and pass
things along.
This is necessary since some drivers won't produce any explicit
modifiers.
We were confusingly printing "supported format" for dmabuf formats
that we end up not adding to our list of supported formats. Don't
do that, it is confusing. At the same time, we shuold print out
the linear formats we support via mmap.
Add an implementation of GdkDmabufDownloader that uses
gsk_renderer_render_texture + GL texture download.
Since gsk isn't threadsafe, we do the download in the main thread,
taking care to not disturb the current GL context of whatever is
going on there at the time.
And since gsk renderers are expensive to create, we cache it
in the display.
Note that gsk does not yet have any special support for
dmabuf textures, so for now, they will always get downloaded
and then reuploaded as GL textures.
