remove relativeResourcePath from gltf resource loading;

all external resources should be added on the Dart side, Filament should never attempt to load these directly from the filesystem.
relativeResourcePath (called gltfPath on the Filament side) is only used for desktop loading, so this is now irrelevant.
Note that Filament must be compiled with GLTFIO_USE_FILESYSTEM=0

thermion_dart/lib/src/filament/src/implementation/ffi_filament_app.dart

@@ -923,8 +923,7 @@ class FFIFilamentApp extends FilamentApp<Pointer> {
     }
 
     var gltfResourceLoader = await withPointerCallback<TGltfResourceLoader>(
-        (cb) =>
-            GltfResourceLoader_createRenderThread(engine, nullptr.cast(), cb));
+        (cb) => GltfResourceLoader_createRenderThread(engine, cb));
 
     final gizmo = await withPointerCallback<TGizmo>((cb) {
       Gizmo_createRenderThread(