Regression testing for rendering

[w23]

We want to check whether code changes affected the final rendered image. Some changes should not affect the final image at all.
For differences we want to figure out whether we broke things, or improved things.

Thus:

• Have a collection of maps/savefiles for distinct rendering features. Make sure that all rendering functionality is covered (diffuse, specular, point, polygon lights, transparency, different objects, etc).
• Have a set of expected screenshots.
  • Figure out which images we need.
• Script to write screenshots
• Utility to compare images
• Script to compare all images
• Script for running the entire test suite.
• Where to store the above?
• How to integrate with GH Actions CI? -> Integrate render tests into CI checks #725

[w23]

Test suite with scripts, saves, and gold images is here: https://rtxash.omgwtf.ru/Half-Life-RTX/HLRTest
The only thing that remains is actions/CI integration

[w23]

Integrating with CI will be done separately when lack of it starts to cause pain.
ref: #725