Add support for manually setting subpixel hinting on outputs.

Many laptop screens report unknown subpixel order. Allow users to manually set subpixel hinting to work around this.

Addresses https://github.com/swaywm/sway/issues/3163

sway/commands/output.c

@@ -18,6 +18,7 @@ static struct cmd_handler output_handlers[] = {
 	{ "res", output_cmd_mode },
 	{ "resolution", output_cmd_mode },
 	{ "scale", output_cmd_scale },
+	{ "subpixel", output_cmd_subpixel },
 	{ "transform", output_cmd_transform },
 };