wayland_shm: Clear busy status when replacing an unassigned buffer

If we render fast enough we can use more than one buffer before the
compositor assigns us buffer ids.  We need to be careful to clear the
busy bit on all but the most recent one.

src/modules/evas/engines/wayland_shm/evas_dmabuf.c

@@ -620,6 +620,7 @@ _evas_dmabuf_surface_post(Surface *s, Eina_Rectangle *rects, unsigned int count,
     * most recently filled unassigned buffer and make sure it gets
     * displayed.
     */
+   if (surface->pre) surface->pre->busy = EINA_FALSE;
    if (!b->wl_buffer)
      {
         surface->pre = b;