Client-side `onAdded` called twice when loading world

[Kneelawk]

The Issue

Currently AbstractParts' onAdded method is called twice on the client-side when the world is loaded or when ever a MultipartBlockEntity is loaded from NBT.

This seems to be caused by the PartContainer.readInitialRenderData first explicitly invoking its parts' onAdded methods and then calling its own validate method which re-invokes the parts' onAdded methods.

Potential Fix

The simplest fix seems to be just removing the explicit invocations of onAdded from the readInitialRenderData method and letting validate handle onAdded instead.

[Kneelawk]

This is probably low-priority. None of my code depends on the client-side onAdded only being called once and I don't think I've seen any other code depend on it either.