Feature/#480_create_a_smaller_test_mesh_for_combustion_demo

closes #480

Starting the combustion demo with the old model took ~70 seconds, now it takes ~10 seconds.

demos/combustion_demo/src/combustion_demo.cpp

@@ -86,7 +86,7 @@ int main(int, char**) {
                             rendering_system, splash, input_system,
                             openvr_system]() {
     auto model_surface_entity = phx::SceneLoader::InsertModelIntoScene(
-        "models/combustion/mixture_surface_0.2_lower.stl", scene.get());
+        "models/combustion/combustion_opt.stl", scene.get());
     auto surface_transform =
         model_surface_entity->GetFirstComponent<phx::Transform>();
     auto surface_material_handle =