`SpringBoneSystem` cannot handle joints with same transform

[TigerHix]

UniVRM/Assets/VRM/Runtime/SpringBone/Logic/SpringBoneSystem.cs

Lines 39 to 42 in ebe8255

	for (int i = 0; i < m_joints.Count; ++i)
	{
	m_jointIndexMap.Add(m_joints[i].Item1, i);
	}

Instead of Add, just use m_jointIndexMap[m_joints[i].Item1] = i here. Otherwise this breaks compatibility with a ton of VRM 0.x models that have joints with the same transform.

Side question: this change was made 5 months ago. Are there any back compatibility tests or regression tests in-place for UniVRM? VRM is now a huge ecosystem, and constantly making changes that break older models is very concerning for the most popular VRM runtime.

[ousttrue]

I hadn't anticipated the same joint appearing multiple times.
I think I could add a unit test for this issue for VRM-0.x

Springbone recognizes duplicated joint registrations and branching as exceptional cases in the configuration.
https://github.com/vrm-c/vrm-specification/blob/master/specification/VRMC_springBone-1.0/README.md#exception

[ousttrue]

In any case, we have no intention of making any changes to 0.x, so if it stops working please report it as a bug.