glx: rework __glXCalculateUsableExtensions to be more readable

We are about to make the GLX extension equation even more complex by
adding force-enable/disable. Before doing so, split the line into
multiple stages that can get a proper comment to explain what is going
on.

The code is taken mostly verbatim from Ian Romanick's comment:
https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7212#note_668045

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>
1 file changed