Vastly Clean Up Rend3 Routine Internals

[cwfitzgerald]

Checklist

• CI Checked:
  • cargo fmt has been ran
  • cargo clippy reports no issues
  • cargo test succeeds
  • cargo rend3-doc has no warnings
  • cargo deny check issues have been fixed or added to deny.toml
• Manually Checked:
  • relevant examples/test cases run
  • changes added to changelog
    • Add credit to yourself for each change: Added new functionality @githubname.

Description

This is a step in the right direction for creating a custom shader example. In general trying to untangle and clean up the rend3_routine api to be less confusing, easier to use, etc.