Investigate dynamic node codegen in MaterialX

[ppenenko]

https://github.com/AcademySoftwareFoundation/MaterialX/blob/main/documents/DeveloperGuide/ShaderGeneration.md#134-dynamic-code-generation

• Step through some relevant examples.
• Does this pattern regenerate the code whenever something changes in the document?
• Can codegen depend on a parameter value known at codegen time?
• Can multiple instances of the same C++ class be registered as separate nodes? Doesn't look that way.

[ppenenko]

See metashade/MaterialX@747bcb4...d9a6687 for some prototype code for generating GLSL code with Metashade at MaterialX codegen time. It involves running a embedded Python interpreter with pybind11.

This is not the top priority now though because Metashade can also be useful for generating target code "statically" - i.e. pre-generating MaterialX XML and respective source code and then using it in MaterialX as an existing library. That is a simpler problem to solve.

At the same time, there are many issues to figure out around generating MaterialX-compatible source (e.g. metashade/metashade#138) which are common between the static and dynamic approaches.

[ppenenko]

#3 addresses the last question in the issue description.