use getNativeHandle for MaterialInstance

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

@@ -321,7 +321,7 @@ class FFIAsset extends ThermionAsset {
   ///
   ///
   @override
-  Future setMaterialInstanceAt(FFIMaterialInstance instance,
+  Future setMaterialInstanceAt(MaterialInstance instance,
       {int? entity = null, int primitiveIndex = 0}) async {
     if (entity != null &&
         !RenderableManager_isRenderable(app.renderableManager, entity)) {
@@ -350,7 +350,7 @@ class FFIAsset extends ThermionAsset {
         Engine_getRenderableManager(app.engine),
         entity,
         primitiveIndex,
-        instance.pointer)) {
+        instance.getNativeHandle())) {
       _logger.warning(
           "Failed to set material instance for entity $entity at primitive index ${primitiveIndex}");
     }

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

@@ -30,7 +30,7 @@ class FFIMaterial extends Material {
   }
 }
 
-class FFIMaterialInstance extends MaterialInstance {
+class FFIMaterialInstance extends MaterialInstance<Pointer<TMaterialInstance>> {
   final Pointer<TMaterialInstance> pointer;
   final FFIFilamentApp app;
 
@@ -198,6 +198,11 @@ class FFIMaterialInstance extends MaterialInstance {
     MaterialInstance_setParameterMat4(
         pointer, name.toNativeUtf8().cast<Char>(), matrix.storage.address);
   }
+
+  @override
+  T getNativeHandle<T>() {
+    return pointer as T;
+  }
 }
 
 extension TStencilFaceExt on StencilFace {

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

@@ -1,3 +1,4 @@
+import 'package:thermion_dart/src/filament/src/interface/native_handle.dart';
 import 'package:thermion_dart/thermion_dart.dart';
 import 'package:vector_math/vector_math_64.dart';
 
@@ -98,7 +99,7 @@ abstract class Material {
   Future destroy();
 }
 
-abstract class MaterialInstance {
+abstract class MaterialInstance<T> extends NativeHandle<T> {
   Future<bool> isStencilWriteEnabled();
   Future setDepthWriteEnabled(bool enabled);
   Future setDepthFunc(SamplerCompareFunction depthFunc);