Model loading-related changes

[AmberIsFrozen]

• Add a constructor for VertexArray which merely copies the value from the other VertexArray
  • This is required to implement copyForMaterialChanges(), a method which makes a shallow copy of the vert arrays with a new MaterialProperties, allowing textures to be changed at runtime for uploaded model.
• Add OptimizedModel.close() to allow closing an OptimizedModel and freeing up resources, which was previously not possible and requires mixin to iterate through VertexArray and closing it.
• Make the constructor public for OptimizedModel to take in an existing VertexArray list.