glx: initial plumbing to let users force-enable/disable extensions

glx: initial plumbing to let users force-enable/disable extensions

This can be useful for debugging or working around bugs such as
Mesa#106 where Wine is expecting to find a visual that isn't
available.

v2:
 - split the indirect GL extension override to its own commit
 - memset the bitfields to 0 in __glXExtensionsCtrScreen

Reviewed-by: Adam Jackson <ajax@redhat.com>
v3:
 - slight rework necessary after splitting the computation of usable
   extensions (Ian)

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Signed-off-by: Martin Peres <martin.peres@mupuf.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7212>