commands/opacity: Call output_configure_scene on updated container

Calling container_update() wasn't enough: If there is no visible window
decorations (title bar, borders) container_update would basically no-op
and the scene wouldn't repaint with the update alpha. By also calling
output_configure_scene() we force a call to
wlr_scene_buffer_set_opacity() thus ensuring we update the scene.

Closes: swaywm#8580

sway/commands/opacity.c

@@ -2,7 +2,8 @@
 #include <stdlib.h>
 #include <strings.h>
 #include "sway/commands.h"
-#include "sway/tree/view.h"
+#include "sway/tree/container.h"
+#include "sway/output.h"
 #include "log.h"
 
 struct cmd_results *cmd_opacity(int argc, char **argv) {
@@ -37,6 +38,7 @@ struct cmd_results *cmd_opacity(int argc, char **argv) {
 	}
 
 	con->alpha = val;
+	output_configure_scene(NULL, &con->scene_tree->node, 1);
 	container_update(con);
 
 	return cmd_results_new(CMD_SUCCESS, NULL);