compositor: Fail on invalid transform for headless

As in aaf35586f4, we want to fail when we are passed an invalid
transform name, not just blindly configure on using the normal
transform. The previous commit missed the callsite from the headless
backend's command-line parsing.

Fix this so that headless fails when an invalid transform is specified
on the command line.

Signed-off-by: Daniel Stone <daniels@collabora.com>

compositor/main.c

@@ -2602,7 +2602,7 @@ load_headless_backend(struct weston_compositor *c,
 	if (transform) {
 		if (weston_parse_transform(transform, &parsed_options->transform) < 0) {
 			weston_log("Invalid transform \"%s\"\n", transform);
-			parsed_options->transform = UINT32_MAX;
+			return -1;
 		}
 		free(transform);
 	}