Implement create_output command for headless backend

sway/commands/create_output.c

@@ -1,4 +1,5 @@
 #include <wlr/config.h>
+#include <wlr/backend/headless.h>
 #include <wlr/backend/multi.h>
 #include <wlr/backend/wayland.h>
 #if WLR_HAS_X11_BACKEND
@@ -17,6 +18,9 @@ static void create_output(struct wlr_backend *backend, void *data) {
 	if (wlr_backend_is_wl(backend)) {
 		wlr_wl_output_create(backend);
 		*done = true;
+	} else if (wlr_backend_is_headless(backend)) {
+		wlr_headless_add_output(backend, 1920, 1080);
+		*done = true;
 	}
 #if WLR_HAS_X11_BACKEND
 	else if (wlr_backend_is_x11(backend)) {
@@ -38,7 +42,7 @@ struct cmd_results *cmd_create_output(int argc, char **argv) {
 
 	if (!done) {
 		return cmd_results_new(CMD_INVALID,
-				"Can only create outputs for Wayland or X11 backends");
+			"Can only create outputs for Wayland, X11 or headless backends");
 	}
 
 	return cmd_results_new(CMD_SUCCESS, NULL);