glx: MakeCurrent fixes.

glx: MakeCurrent fixes.

1) If MakeContextCurrent is called with (NULL, None, None), Don't
   send the request to the X server if the current context is direct.
2) Return BadMatch in some error cases according to the glx spec.
3) If MakeContextCurrent is called for a context which is current in
   another thread, return BadAccess according to the glx spec.

Signed-off-by: Thomas Hellstrom <thellstrom-at-vmware-dot-com>