Arnold ShaderNetworkAlgo : Add custom substitutions API

[johnhaddon]

This is an API I promised to @tomc-cinesite and @alex-savenko-at-cinesite, to facilitate the creation of a neat system of hierarchical shader overrides. I'm really not sure that these substitutions belong in the renderer backend, and think there is a good case that they belong in the shader publishing process instead. But I have done the work in order that no one can accuse me of not wanting to do the work, and to thus strengthen my position in any future negotiations :)

[boberfly]

Just to note its a mess over on the other DCC app where <attr:something> works for Arnold like it does in Gaffer, but Prman uses <primstr:something> so I like the idea of it living outside the renderer backend...

[johnhaddon]

Are you saying you'd like to see this lifted even further out @boberfly? So that we did it before even passing to the renderer? I was wondering about that when doing this. I went for the smallest possible change this time around, but I think moving it all up a level and caching the CompoundObject -> Renderer::AttributesInterface conversion might be worth thinking about.

[boberfly]

Yeah I am just worried you get into a world of inconsistency between the renderers doing it this way, like if you plan on using the same shaders between renderers (might be fairly niche though in practice). I do enjoy Gaffer's overall consistency everywhere else, but I don't know what the bigger goal is with this API. The <attr:something> seems to work well for string substitutions between the renderers.