Support a library of npr or modeling definitions

[bernardkwok]

1.39 Proposal for non-pbr library

Placeholder for proposal to add in a new core library to support non-pbr shading graphs.

1. This includes the idea to support an "unlit" shader which allows for
   a. Any upstream non-pbr graph to be connected upstream. e.g. a toon or gooch graph, or if unconnected this just becomes a constant colour/value shader.
   b. connection to surfacematerial nodes and thus can be used for material assignment as well as being associated with a displacement shader.
   c. No special case "sampling" nodes need to be attached for code generation of upstream graphs. Currently an must be added and for OSL code generation an addition emissive shader must be added for unit testing (which isn't strictly correct).

   Note that a constant edf node to a surface node is the base functional nodegraph definition. Some codegen efficiencies can be made as needed. e.g. color variant is shown:
   

2. A core set of functional nodegraphs for desired effects.

Note: at this point we are not considering shaders which may require additional stages in hardware e.g. a stippled edge shader.

[bernardkwok]

Re-open as current implementation needs to be revisited.