scene: use transparent black as background color

This is useful when a compositor sets up a scene-graph for
capturing an individual toplevel.

wlr_output picks XRGB by default and only falls back to ARGB if
the driver doesn't support XRGB, so this shouldn't have an impact
on the result in most cases.

types/scene/wlr_scene.c

@@ -2353,7 +2353,7 @@ bool wlr_scene_output_build_state(struct wlr_scene_output *scene_output,
 
 	wlr_render_pass_add_rect(render_pass, &(struct wlr_render_rect_options){
 		.box = { .width = buffer->width, .height = buffer->height },
-		.color = { .r = 0, .g = 0, .b = 0, .a = 1 },
+		.color = { .r = 0, .g = 0, .b = 0, .a = 0 },
 		.clip = &background,
 	});
 	pixman_region32_fini(&background);