gl-renderer: Don't change the region supplied to output repaint

The core uses this region to clear from the primary plane damage the
area that was repainted. If we add the old buffer damage to that, it
may end up clearing more damage from the primary plane than it was
intended.

src/gl-renderer.c

@@ -957,6 +957,7 @@ gl_renderer_repaint_output(struct weston_output *output,
 	EGLBoolean ret;
 	static int errored;
 	int32_t width, height, i;
+	pixman_region32_t total_damage;
 
 	width = output->current->width +
 		output->border.left + output->border.right;
@@ -987,10 +988,13 @@ gl_renderer_repaint_output(struct weston_output *output,
 				      &go->buffer_damage[i],
 				      output_damage);
 
-	pixman_region32_union(output_damage, output_damage,
-			      &go->buffer_damage[go->current_buffer]);
+	pixman_region32_init(&total_damage);
+	pixman_region32_copy(&total_damage,
+			     &go->buffer_damage[go->current_buffer]);
 
-	repaint_surfaces(output, output_damage);
+	repaint_surfaces(output, &total_damage);
+
+	pixman_region32_fini(&total_damage);
 
 	if (gr->border.texture)
 		draw_border(output);