> Running automated regression-tests against a library of known-accurate titles actually sounds extremely practical.

Dolphin (the GameCube/Wii emulator) already does this, actually. They make dumps of various states of the graphics pipeline across multiple revisions of the source and generate image-based diffs of the results.

https://fifoci.dolphin-emu.org/about/

Here's an example.

https://fifoci.dolphin-emu.org/compare/5108854-5102201/