Ensure GLX context is set for OSD init.

Should make OSD working again.
Clamp GLX OSD width & height to OSD size

This is actually a port of VA-API commit
aed7271f2185a0e0bf061308034cec6ca58966e8 to VA-API-GLX side.

Should fix some ('broken'?) skins like skinFlatPlus to not do invalid
drawing operations.