drisw: Use separate drisw_loader_funcs for shm

drisw: Use separate drisw_loader_funcs for shm

The original code was modifying the global drisw_lf variable, which is bad
when there are multiple contexts in single process, each initialized with
different loader. One may support put_image_shm and the other not.

Since there are currently only two possible combinations, lets create two
global tables, one for each. Lets make them const, since we won't change them
and they can be shared.

This fixes crash in VLC. It used two GL contexts (each in different thread), one
was initialized by its Qt GUI, the other by its video output plugin. The first
one set the put_image_shm=drisw_put_image_shm, the second did not, but
since the same structure was used, the drisw_put_image_shm was used too. Then
it crashed because the second loader did not have putImageShm set.

Downstream bug:
https://bugzilla.opensuse.org/show_bug.cgi?id=1113533

v2: Added Fixes and described the VLC bug.

Fixes: 63c427fa71a ("drisw: use putImageShm if available")
Signed-off-by: Michal Srb <msrb@suse.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>