apprt/gtk-ng: assert that GTK is uninitialized in setGtkEnv

Signed-off-by: Tristan Partin <tristan@partin.io>

src/apprt/gtk-ng/class/application.zig

@@ -541,6 +541,8 @@ pub const GhosttyApplication = extern struct {
 ///
 /// This must be called BEFORE GTK initialization.
 fn setGtkEnv(config: *const Config) error{NoSpaceLeft}!void {
+    assert(gtk.isInitialized() == 0);
+
     var gdk_debug: struct {
         /// output OpenGL debug information
         opengl: bool = false,