Circular reference with branch materials

[Dragorn421]

A material A may branch to an object B that itself includes material A.
A material A may branch to an object D that has another material C branching to another object B which includes material A.
and so on

This causes a circular reference. objex2 should detect those and abort the export

In the mean time, z64convert 1.0.2 detects the issue

https://discord.com/channels/891705295640870922/891707377173266462/917707697858691072