build: make opengl-old video output optional

All my systems have new enough mesa: opengl VO works fine with both r600 and nouveau drivers. So i would like to disable opengl-old VO at build time.
```
# time USE='opengl-old' MAKEOPTS="-j1" emerge -1vb --nodeps mpv;du -s /usr/bin/mpv 
'build' finished successfully (4m25.780s)
real    6m15.809s
user    5m44.179s
sys     0m40.105s
1345    /usr/bin/mpv

# time USE='-opengl-old' MAKEOPTS="-j1" emerge -1vb --nodeps mpv;du -s /usr/bin/mpv
'build' finished successfully (4m15.652s)
real    6m4.894s
user    5m34.077s
sys     0m39.904s
1308    /usr/bin/mpv
```