USD and MaterialX direction

USD is a possible authored scene-meaning layer and MaterialX a possible portable material-description edge. Neither is part of the shipped Shader Toy or Render Toy workflow in 0.3.0.

Any future integration should let USD own authored scene meaning and use established change propagation rather than adding a parallel Workbench scene database. ShaderDocument remains an independent shader-authoring product unless an explicit consumer edge is designed and tested.