Type description improvements

[niklasharrysson]

This change list is an overhaul of the type system in shader generation.

Improvements include:

• Removing the singleton pattern for TypeDesc storage and access, making the type system thread safe.
• Improved handling of TypeDesc internal data, supporting any amount of data for the future, while keeping the class size minimal.
• Handling of re-registration of type descriptions.
• Support for keeping type descriptions alive after the lifetime of ShaderGenerator and GenContext. For applications that holds/uses the ShadePort reflection data that is generated by shader generation, even after the ShaderGenerator itself is destroyed. (To do this an application can create and pass in an external TypeSystem).

Co-work with @ld-kerley

[niklasharrysson]

This PR includes and replaces #2107, which can be closed if this PR is approved.

[jstone-lucasfilm]

@niklasharrysson @ld-kerley Since we're encouraging developers to use the associated create function for each ShaderGenerator subclass, I would recommend omitting default arguments for our ShaderGenerator constructors, leaving only the default arguments on our ShaderGenerator create functions. This would simplify the code a bit, and would help to clarify which entry point developers are expected to use.

Does that sound like a reasonable simplification to you both?

[niklasharrysson]

Sounds good! I’ll make that change 👍