Wayland: Add environment variable to disable libdecor loading

Libdecor has some quirks and depending on the setup it might be useful
to switch to the xdg-shell branch. Recompiling the whole engine without
libdecor is not an acceptable way to do that.

platform/linuxbsd/wayland/wayland_thread.cpp

@@ -3739,10 +3739,16 @@ Error WaylandThread::init() {
 	}
 #endif // SOWRAP_ENABLED
 
-	if (libdecor_found) {
-		libdecor_context = libdecor_new(wl_display, (struct libdecor_interface *)&libdecor_interface);
+	bool skip_libdecor = OS::get_singleton()->has_environment("GODOT_WAYLAND_DISABLE_LIBDECOR");
+
+	if (skip_libdecor) {
+		print_verbose("Skipping libdecor check because GODOT_WAYLAND_DISABLE_LIBDECOR is set.");
 	} else {
-		print_verbose("libdecor not found. Client-side decorations disabled.");
+		if (libdecor_found) {
+			libdecor_context = libdecor_new(wl_display, (struct libdecor_interface *)&libdecor_interface);
+		} else {
+			print_verbose("libdecor not found. Client-side decorations disabled.");
+		}
 	}
 #endif // LIBDECOR_ENABLED