cogl: Remember to bind the GLX context to our drawable before querying

Or else glXQueryDrawable will fail per the GLX_EXT_buffer_age spec:

If querying GLX_BACK_BUFFER_AGE_EXT and draw is not bound to the calling thread's current context a GLXBadDrawable error is generated.

This mistake went unnoticed until mtk_x11_error_trap_push was introduced (55e3b2e5) because for some reason it is incapable of trapping glXQueryDrawable. Prior to that it seems cogl_onscreen_glx_get_buffer_age would trap and so always returned zero.

This means we're reenabling clipped redraws on X11 here for the first time in a long time.

Closes: #3007 (closed)