vo_opengl: don't use 10 bit video on Intel GPUs

vo_opengl was originally written against OpenGL 3 core, and it seems
GPUs/drivers supporting this are mostly sane. Later, it was made to work
with OpenGL 2.1 too. Lately we removed the requirement for RG textures,
and look, someone reported a problem with "lesser" Intel GPUs.

This commit does the same in vo_opengl what was added to vo_opengl_old a
long time ago.

Fixes #1383.