gtk(wayland): fallback when on-demand mode isn't supported

This shouldn't be a real problem anymore since as of now (May 2025)
all major compositors support at least version 4, but let's do this
just in case.

pkg/gtk4-layer-shell/src/main.zig

@@ -27,6 +27,10 @@ pub fn isSupported() bool {
     return c.gtk_layer_is_supported() != 0;
 }
 
+pub fn getProtocolVersion() c_uint {
+    return c.gtk_layer_get_protocol_version();
+}
+
 pub fn initForWindow(window: *gtk.Window) void {
     c.gtk_layer_init_for_window(@ptrCast(window));
 }