Lambert instancing example: replace ShaderMaterial with modified MeshLambertMaterial

[WestLangley]

This is a follow-up to #14012.

Unfortunately, the WebGLRenderer code injections that support texture encoding only work properly for built-in materials -- not ShaderMaterial (unless the user jumps through hoops to define material setters/getters for each ShaderMaterial uniform).

Consequently, rather than adding Lambert shader chunks into a custom ShaderMaterial (as in #14012), it is preferable to add custom instancing code into the built-in MeshLambertMaterial, instead.

This is all quite tedious.

[mrdoob]

This is all quite tedious.

Indeed... It's good exploration to know how it could be done inside core though!

[mrdoob]

Thanks!

[pailhead]

Why is only Lambert considered for instancing examples?

There is a demo with phong and standard working too.

@WestLangley

Consequently, rather than adding Lambert shader chunks into a custom ShaderMaterial (as in #14012), it is preferable to add custom instancing code into the built-in MeshLambertMaterial, instead.

What's the "built-in" MeshLambertMaterial. I'm really confused about this.
#14098

Should be doing exactly what you are describing.

I'm not sure which version this is, but should this be the example of how this is done?

Modifying the global THREE.ShaderLib may possibly be not a good practice? However, if one opts to modify, there should be less code if THREE.ShaderChunk was modified instead. It would also work on other materials, not just lambert.

Not sure why onBeforeCompile exists if this is the three.js pattern?