Improved shader graph support

[Cameron-Micka]

Describe the problem

Graphics Tools supports Unity's Universal Render Pipeline (URP) but does not expose functionality to the new Shader Graph that comes with URP.

Describe the solution you'd like

Graphics Tools should expose a library of common functions for effects it supports via custom shader graph nodes. This will allow users to make more bespoke shaders and effects.

Describe alternatives you've considered

Not use Shader Graph and continue to use the uber shader approach.

Additional context

See Graphics Tools for Unreal for example of how methods are exposed to a shader graph: https://github.com/microsoft/MixedReality-GraphicsTools-Unreal

[jayliu50]

This would be huge. Either shader graph support or break out functionality to separate hlsl files. Right now I don't see a clean way of extending upon existing functionality. (example, I want my own custom shader to be clipped the same way the super shader is clipped)