swaybar: set opaque region

When the background color is fully opaque, set the surface's opaque
region to the whole surface.

swaybar/render.c

@@ -831,6 +831,15 @@ void render_frame(struct swaybar_output *output) {
 		wl_surface_damage(output->surface, 0, 0,
 				output->width, output->height);
 
+		uint32_t bg_alpha = ctx.background_color & 0xFF;
+		if (bg_alpha == 0xFF) {
+			struct wl_region *region =
+				wl_compositor_create_region(output->bar->compositor);
+			wl_region_add(region, 0, 0, INT32_MAX, INT32_MAX);
+			wl_surface_set_opaque_region(output->surface, region);
+			wl_region_destroy(region);
+		}
+
 		struct wl_callback *frame_callback = wl_surface_frame(output->surface);
 		wl_callback_add_listener(frame_callback, &output_frame_listener, output);
 		output->frame_scheduled = true;