add setMaterialInstanceAt to FilamentApp interface

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

@@ -557,6 +557,15 @@ class FFIFilamentApp extends FilamentApp<Pointer> {
     return instance;
   }
 
+  ///
+  ///
+  ///
+  Future setMaterialInstanceAt(ThermionEntity entity, int index,
+      MaterialInstance materialInstance) async {
+    RenderableManager_setMaterialInstanceAt(renderableManager, entity, index,
+        (materialInstance as FFIMaterialInstance).pointer);
+  }
+
   ///
   ///
   ///

thermion_dart/lib/src/filament/src/interface/filament_app.dart

@@ -196,7 +196,13 @@ abstract class FilamentApp<T> {
   ///
   ///
   Future<MaterialInstance> getMaterialInstanceAt(
-      ThermionEntity entity, int index);
+      ThermionEntity entity, int primitiveIndex);
+
+  ///
+  ///
+  ///
+  Future setMaterialInstanceAt(
+      ThermionEntity entity, int primitiveIndex, MaterialInstance materialInstance);
 
   ///
   ///