xquartz/glx: Error out for MakeContextCurrent(draw != read)

CGL doesn't have a way to express this directly, unlike EGL WGL and GLX.
It might be implementable, but it's never actually worked, and it's a
fairly niche feature so we're better off throwing an error if someone
attempts it.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Jeremy Huddleston Sequoia <jeremyhu@apple.com>
Signed-off-by: Adam Jackson <ajax@redhat.com>

hw/xquartz/GL/indirect.c

@@ -387,6 +387,9 @@ __glXAquaContextMakeCurrent(__GLXcontext *baseContext)
 
     GLAQUA_DEBUG_MSG("glAquaMakeCurrent (ctx 0x%p)\n", baseContext);
 
+    if (context->base.drawPriv != context->base.readPriv)
+        return 0;
+
     if (attach(context, drawPriv))
         return /*error*/ 0;