backend/drm: check whether clipped damage is empty

We were checking whether the damage region was empty before
clipping. However a non-empty damage region can become empty after
clipping. Instead, check whether the clipped region is empty.

Fixes: 4339c37f99 ("backend/drm: clip FB damage")

backend/drm/atomic.c

@@ -156,18 +156,20 @@ static bool create_gamma_lut_blob(struct wlr_drm_backend *drm,
 
 bool create_fb_damage_clips_blob(struct wlr_drm_backend *drm,
 		int width, int height, const pixman_region32_t *damage, uint32_t *blob_id) {
-	if (!pixman_region32_not_empty(damage)) {
-		*blob_id = 0;
-		return true;
-	}
-
 	pixman_region32_t clipped;
 	pixman_region32_init(&clipped);
 	pixman_region32_intersect_rect(&clipped, damage, 0, 0, width, height);
 
 	int rects_len;
 	const pixman_box32_t *rects = pixman_region32_rectangles(&clipped, &rects_len);
-	int ret = drmModeCreatePropertyBlob(drm->fd, rects, sizeof(*rects) * rects_len, blob_id);
+
+	int ret;
+	if (rects_len > 0) {
+		ret = drmModeCreatePropertyBlob(drm->fd, rects, sizeof(*rects) * rects_len, blob_id);
+	} else {
+		ret = 0;
+		*blob_id = 0;
+	}
 	pixman_region32_fini(&clipped);
 	if (ret != 0) {
 		wlr_log_errno(WLR_ERROR, "Failed to create FB_DAMAGE_CLIPS property blob");