opengl: disable software rasterizers by default
Mirroring logic from mpv's vo_gpu. Motivated by the fact that many
GPU-less build environments have barely functioning or broken software
rasterizer fallbacks. The status of this bool is controlled explicitly
for the CI by the new CI_ALLOW_SW
define, to allow gitlab tests to
continue using (new enough versions of) llvmpipe.
Closes #109 (closed)