Material shader extensions

[pailhead]

A proposal that allows an alternate approach to extending built-in materials. Only the core change is present in this PR.

Some examples are made in #14206, click on the images to see the demos.

1. Use this API in GLTFLoader to extend the StandardMaterial with spec/gloss model. And then in the actual loader example, make an "inline" extension for simple instancing (instancing_lambert example). The goal is to have two completely agnostic extensions that just work together - the instancing is applied to an arbitrary material, while the spec/gloss is intended solely for StandardMaterial.
   

2. Combine three different extensions (simple instancing, per map uv channels and spec/gloss) on a single material, and modify one of the extensions "inline" (use the instancing offset, but animate the scale effect in the shader). The demo uses a refactored GLTFLoader that just calls one of these decorators, the rest are applied in the inline demo code.
   
   

Individually these can be done with onBeforeCompile, but onBeforeCompile may possibly have issues. The name might be better as onBeforeParse or onWillRefresh. Combining these is a challenge with it though, while this API may be more flexible, if not the friendliest one.

Per map transforms with onBeforeCompile.
Instancing lambert refactored with onBeforeCompile
Standard/gloss refactor with onBeforeCompile.

This should be decoupled from /examples/nodes and not be blocked by it. Thank you.

[pailhead]

This proposal and all the ones before it should, under no circumstances be used to add features to the library.

concerns about shader patches to the builtin materials as a means of, or replacement for, adding features to the library

The only code that this PR should ever add to three.js from here to eternity is the code for itself. It should never ever directly be used to add a feature to the library. It is by itself a feature of the library, and it should never ever go past that.

It should only be used by users to add features to their applications. I hope this clarifies years of confusion. :) I'll try to write this more legalese.

I've used the GLTFLoader example because that class is in examples and simulates someone having their own app. Someone developing AFrame probably has to rely on GLTFLoader, i myself have never ever used it, so the need for spec gloss, Khronos group extensions, instancing and serialization, are different between our apps.

This is conceptually totally different from NodeMaterial there you actually do want to integrate it into the core and base other features on top of it.

ping @mrdoob ping @donmccurdy

[mrdoob]

#14245 (comment)