fix instancing so that gltf assets always fill the first instance slot

thermion_dart/test/instancing_tests.dart

@@ -0,0 +1,40 @@
+@Timeout(const Duration(seconds: 600))
+import 'package:test/test.dart';
+import 'package:vector_math/vector_math_64.dart';
+import 'helpers.dart';
+
+void main() async {
+  final testHelper = TestHelper("instancing");
+  await testHelper.setup();
+  test('gltf assets always create one instance', () async {
+    await testHelper.withViewer((viewer) async {
+      var asset =
+          await viewer.loadGltf("file://${testHelper.testDir}/assets/cube.glb");
+      expect(await asset.getInstanceCount(), 1);
+    });
+  });
+
+  test('create gltf instance', () async {
+    await testHelper.withViewer((viewer) async {
+      await viewer
+          .loadIbl("file://${testHelper.testDir}/assets/default_env_ibl.ktx");
+      await viewer.loadSkybox(
+          "file://${testHelper.testDir}/assets/default_env_skybox.ktx");
+      await viewer.setPostProcessing(true);
+      await viewer.setAntiAliasing(false, true, false);
+
+      var asset = await viewer.loadGltf(
+          "file://${testHelper.testDir}/assets/cube.glb",
+          numInstances: 2);
+
+      await testHelper.capture(viewer.view, "gltf");
+      var instance = await asset.createInstance();
+      await viewer.addToScene(instance);
+      print(instance.entity);
+      print(await instance.getChildEntities());
+
+      await instance.setTransform(Matrix4.translation(Vector3(1, 0, 0)));
+      await testHelper.capture(viewer.view, "gltf_with_instance");
+    });
+  });
+}