fix: make on* callbacks settable

[andretchen0]

Problem

on* callbacks – e.g., onBeforeRender – are methods on THREE Objects. In plain THREE, these can be overridden by users.

Currently in TresJS, a prop like on-before-render="..." calls the default function – which does nothing:

• https://github.com/mrdoob/three.js/blob/a2e9ee8204b67f9dca79f48cf620a34a05aa8126/src/materials/Material.js#L105
• https://github.com/mrdoob/three.js/blob/a2e9ee8204b67f9dca79f48cf620a34a05aa8126/src/core/Object3D.js#L123

It should instead set the function.

Solution

Add a predicate in src/core/nodeOps.ts where, for prop-name="value":

• prop-name starts with on
• object[propName] is a function
• value is a function

In this limited case, value will replace object[propName].

A unit test and a playground were also added.

Context

This is a more targeted approach than R3F. There's a good chance that this could be absorbed into a more general predicate in a future refactor.

But the predicate is kept tight here to avoid unexpected behavior elsewhere.

Closes #360

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