glx: Make sure that DRI2/swrast calls haven't changed the GL context.

glx: Make sure that DRI2/swrast calls haven't changed the GL context.

These functions are called from the GL driver, in some series of GL
calls by GLX.  If some server component (like glamor CreatePixmap for
GetBuffers()) changes the GL context on us, we need to set it back or
the later GL calls will land in the glamor context instead of the GLX
context.

Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>