don't share a single material instance for all highlighted objects

destroy highlight material instances on highlight asset destroyed
expose highlight instances on Scene via getAssetForHighlight

thermion_dart/test/scene_tests.dart

@@ -24,6 +24,7 @@ void main() async {
 
       await testHelper.capture(viewer.view, "stencil_highlight");
       await scene.removeStencilHighlight(cube);
+
       await testHelper.capture(viewer.view, "stencil_highlight_removed");
     }, createStencilBuffer: true);
   });