xlib: fix single buffer window resize bug

When a single-buffered window was resized the new window size was never
detected.  This fix that, but there's still a bug which causes window
contents corruption for certain window sizes...

src/gallium/winsys/xlib/xlib_softpipe.c

@@ -303,6 +303,7 @@ xm_flush_frontbuffer(struct pipe_winsys *pws,
     */
    XMesaContext xmctx = (XMesaContext) context_private;
    xlib_softpipe_display_surface(xmctx->xm_buffer, surf);
+   xmesa_check_and_update_buffer_size(xmctx, xmctx->xm_buffer);
 }