shm: clear buffer’s dirty region before returning a cached buffer

A non-busy buffer should always have an empty dirty region

shm.c

@@ -250,6 +250,7 @@ shm_get_buffer(struct wl_shm *shm, int width, int height, unsigned long cookie,
                         cookie, (void *)&it->item);
                 it->item.busy = true;
                 it->item.purge = false;
+                pixman_region32_clear(&it->item.dirty);
                 free(it->item.scroll_damage);
                 it->item.scroll_damage = NULL;
                 xassert(it->item.pix_instances == pix_instances);