Extract backend from standard.js

[slimbuck]

This PR contains the first round of changes toward restructuring standard material and its chunks. (For background see #4250).

Summary of changes:

• extract the material backend chunk-building code from standard.js into lit-shader.js
• modify chunks into strict frontend and backend
• reorder runtime chunks so all frontend code executes together (some frontend and backend chunks were previously interleaved)
• remove toksvig specular AA (see Toksvig specular doesn't work #4260)

Notes:

• these changes have been tested with test-server and engine examples
• a followup PR will address the material options driving LitShader, which is currently a monolith that includes all options for frontend and backend

Questions:

• is the change to emissivePS worth the breakage? cc @mvaligursky @Maksims - YES

Changes to frontend chunks

chunk	end	changes
clearCoatNormalPS	front	refrain from generating world CC reflection, now done on the backend instead normalize final world space normal
diffusePS	front	Fix gamma handling relative to albedo detail
diffuseDetailMapPS	front	Gamma correct detail map before combining with base albedo
emissivePS	front	set dEmissive global instead of returning the value in order to bring it in line with the other frontend components
lightmapSingleVert.js	front	removed (unused)
lightmapDirPS, lightmapSinglePS	front	renamed the lightmap function to getLightMap() instead of addLightMap() changed the implementation to write dLightmap and dLightmapDir global instead of updating dDiffuseLight and dSpecularLight directly backend now handles combining lightmap in lightmapAddPS and lightmapDirAddPS
normalMapFastPS	front	removed
normalMapPS	front	added MAPTEXTURE #define like the other chunks normalize final normal when normal texture isn't defined, calculate normal from geometry normal instead
normalDetailMapPS	front	remove two (mostly) unnecessary calls to normalise - final normal is normalized instead
clusteredLightPS	back	remove dead code
endPS	back	combine emissive with dEmissive instead of a call to getEmission()
lightmapAddPS, lightmapDirAddPS	back	new chunks for adding the lightmap values passed in from the backend
lightSpecularAnisoGGXPS	back	added clear coat #define
lightSpecularBlinnPS, lightSpecularPhongPS	back	added clear coat #define, removed call to antiAliasGlossiness()
normalVertexPS	back	removed chunk, moved functionality to normalMapPS frontend chunk
specularAaNonePS, specularAaToksvigPS, specularAaToksvigFastPS	back	removed
startPS	back	removed global declarations, generate them on demand instead

[slimbuck]

@kungfooman that is fixed now - I really appreciate you taking a look!

[mvaligursky]

few small comments, otherwise great work!