Blender Git Loki

Cleanup: Store instances id attribute with other attributes

Now that we can store any dynamic attribute on the instances component,
we don't need the special case for `id`, it can just be handled by the
generic attribute storage. Mostly this just allows removing a bunch
of redundant code.

I had to add a null check for `update_custom_data_pointers` because
the instances component doesn't have any pointers to inside of
custom data.

Differential Revision: https://developer.blender.org/D13430