Get an attribute accessor to retrieve or modify data of give attribute
Kind: instance method of Mesh Returns: MeshAttributeAccessor - attr Attribute to get access to or null,
if mesh does not have this attribute.
If there are no shaders in the scene that use TextureCoordinate for example,
no meshes will have the TextureCoordinate attribute.
For flexible reusable components, take this into account that only Position
is guaranteed to be present at all time.