st/dri: update dri2 drawables when viewport is changed

Fixes gnome-shell on nouveau, as well as window resize with various
other applications.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

src/gallium/include/pipe/p_screen.h

@@ -266,6 +266,11 @@ struct pipe_screen {
 
    void (*video_surface_destroy)( struct pipe_video_surface *vsfc );
 
+   /**
+    * Do any special operations to ensure buffer size is correct
+    */
+   void (*update_buffer)( struct pipe_screen *ws,
+                          void *context_private );
 
    /**
     * Do any special operations to ensure frontbuffer contents are