title: Forge Viewer material manager getModelMaterials() doesn't return added materials
body: <p>When I add a new material using <code>materialManager.addMaterial(...)</code> and afterwards, call <code>materialManager.getMaterials()</code>, the returned map doesn't include my added material. If I want to reuse my material, I have to get it from <code>materialManager._materials</code>. Is there a reason <code>materialManager.getMaterials()</code> doesn't return user added materials?</p>