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.
diff --git a/src/gl-renderer.c b/src/gl-renderer.c
index a5dc2f3..4c1bc23 100644
--- a/src/gl-renderer.c
+++ b/src/gl-renderer.c
@@ -957,6 +957,7 @@
 	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 @@
 				      &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);