Add easy way to set global constants by tag to be used when rendering predicates

[subsoap]

This project has a slightly modified version with a not as fully featured as necessary version of this issue.

It's useful to set global variables you want many models to be able to use for things such as lighting without adding extra drawcalls by setting them individually. Probably want a solution which can be used by all render predicate tags individually.

https://github.com/subsoap/2021_shader_experiments