NodeMaterial structure changes compared to existing shaders

[pailhead]

Trying to extract various topics from #7522

See this comment for a direction for texturing/mapping imposed by NodeMaterial. #14214 (comment)

I propose this requirement be extracted from NodeMaterial and considered for application to the other systems:

For example, right now the templates are limited by this pattern:
https://github.com/mrdoob/three.js/blob/dev/src/renderers/shaders/ShaderChunk/alphamap_fragment.glsl

//alphamap_fragment.glsl

#ifdef USE_ALPHAMAP

	diffuseColor.a *= texture2D( alphaMap, vUv ).g; // why hardcode "vUv" into the coord slot?

#endif

It can be refactored to be more in line with the need imposed by NodeMaterial.

//alphamap_fragment.glsl

#ifdef USE_ALPHAMAP

	diffuseColor.a *= texture2D( alphaMap, alphaUV ).g;

#endif

To be preceded by:

//alphamap_mapping_fragment.glsl

#ifdef USE_ALPHAMAP
    vec2 alphaUV = vUv;
#endif

This would break up and decouple a chunk that is overloaded to begin with IMO, which would then validate the comparison with NodeMaterial.

Would it make sense to document these differences before the new API becomes default?
Would it also make sense to refactor the existing structure, given that #7522 is still not in /src while the templates are? Ie. is there a reason to keep the templates as is, other than this PR from 2015?

Why keep the current template convoluted?

[mrdoob]

#14245 (comment)