[unity] UpdateWhenInvisible incorrect when using SkeletonRenderSeparator

[HaraldCsaszar]

Reported on the forum:
https://esotericsoftware.com/forum/d/26099-problem-with-update-when-invisible/9
Reproduction project sent via email:
FMfcgzGxSvBlvgmBxGQLHGvHlRlzGtMW

Problem is that SkeletonRenderer OnBecameInvisible and OnBecameVisible are no longer triggered when SkeletonRenderSeparator disables the main MeshRenderer and uses multiple SkeletonPartsRenderers instead.

Each SkeletonPartsRenderer would require to capture OnBecameInvisible and OnBecameVisible and at the SkeletonRenderSeparator it needs to be decided if all are invisible or any part is visible, then SkeletonRenderer.OnBecameInvisible and .OnBecameVisible should be called.