message: gltfio: generate tangents if requested by the material
  fixes
diff: @@ -129,7 +129,7 @@ struct FAssetLoader : public AssetLoader {
  FFilamentInstance* instance);
  void createRenderable(const cgltf_node* node, Entity entity, const char* name);
  bool createPrimitive(const cgltf_primitive* inPrim, Primitive* outPrim, const UvMap& uvmap,
  - const char* name);
  + const char* name, MaterialInstance* mi);
  void createLight(const cgltf_light* light, Entity entity);
  void createCamera(const cgltf_camera* camera, Entity entity);
  MaterialInstance* createMaterialInstance(const cgltf_material* inputMat, UvMap* uvmap,
  @@ -469,7 +469,7 @@ void FAssetLoader::createRenderable(const cgltf_node* node, Entity entity, const
  builder.material(index, mi);
  // Create a Filament VertexBuffer and IndexBuffer for this prim if we haven't already.
  - if (!outputPrim->vertices && !createPrimitive(inputPrim, outputPrim, uvmap, name)) {
  + if (!outputPrim->vertices && !createPrimitive(inputPrim, outputPrim, uvmap, name, mi)) {
  mError = true;
  continue;
  }
  @@ -532,7 +532,7 @@ void FAssetLoader::createRenderable(const cgltf_node* node, Entity entity, const
  }
  bool FAssetLoader::createPrimitive(const cgltf_primitive* inPrim, Primitive* outPrim,
  - const UvMap& uvmap, const char* name) {
  + const UvMap& uvmap, const char* name, MaterialInstance* mi) {
  outPrim->uvmap = uvmap;
  // Create a little lambda that appends to the asset's vertex buffer slots.
  @@ -678,7 +678,8 @@ bool FAssetLoader::createPrimitive(const cgltf_primitive* inPrim, Primitive* out
  }
  // If the model is lit but does not have normals, we'll need to generate flat normals.
  - if (inPrim->material && !inPrim->material->unlit && !hasNormals) {
  + auto const* const material = mi->getMaterial();
  + if (material->getRequiredAttributes().test(VertexAttribute::TANGENTS) && !hasNormals) {
  vbb.attribute(VertexAttribute::TANGENTS, slot, VertexBuffer::AttributeType::SHORT4);
  vbb.normalized(VertexAttribute::TANGENTS);
  cgltf_attribute_type atype = cgltf_attribute_type_normal;
  