Add NodeGraph "Flattening" Utilities

[bernardkwok]

This is a WIP proposal for how to provide utilities for "flattening" graphs within MaterialX.

Criteria:

• The ability to choose which "roots" within a graph to flatten.

  • one or more selected outputs
  • a selected node or a selected subset of nodes
  • selection based on an attribute, or metadata. (e.g. based on "target")

• The choice of flattening

  • just a node
  • a node + all upstream nodes.
  • just upstream nodes.

• The ability to flatten nodes which are implemented as nodegraphs. This can even include "standard library" definitions:

  • Note that this is a desired ability to have for creating a copy of an implementation for "publishing" workflows. e.g. "glossiness_anisotropy" "flattened"

• If a node has attributes / metadata on parent nodes then there should be the option to pass these on to the flattened nodes.

• The ability to flatten "in-place" or create extracted copies of "flattened" nodes.

  • The extraction process should take into account upstream data inputs such as geometry streams.

• It is possible to add a "action" hook for each sub-graph to flatten. For example a shader generation "hook" could be provided where a given node is selected and to create the shader for all upstream nodes a temporary output is created to perform sampling with.

Note that this flattening logic is complimentary to existing image "baking" utilities by adding baking a the "action".

Note that flattening logic can be hooked into "reference" implementation generation as well (currently no implementations are output for nodegraph implementations).

[bernardkwok]

Note that #1013 adds in a predicate for selective flattening, but still needs to start from a nodegraph (not node).