Adaptive CRT shader compatibility fixes for really old GPUs

[johnnovak]

Description

Fixes #3114

@Burrito78 kindly gave me remote desktop access to his problematic old MacMini with an Intel HD 4000 iGPU that displays only a black screen when using any of the new adaptive CRT shaders. I was able to pinpoint the problem by the time-tested method of trial and error, also known as "just dicking around" in some circles 😎

The problem seems to be that there's just too much branching going on in one of the functions. GLSL is still a fairly high-level language at the end of the day, and the shader compilers try to do their best to optimise per-pixel branching away. This branching code is not a problem at all on more recent GPUs (7-8 years old), but on this ancient 10+ year old iGPU, the shader compiles without errors, but somehow, who knows how and why, it just displays a black screen at runtime... Commenting out some of the branches fixes it, so the solution I adopted is to comment every branch of the function out that is not used by the adaptive shaders. This is an attempt to pre-empt similar problems on other weird old GPUs.

For the record, the problem could not be reproduced on my $200 bottom-of-the-barrel mini laptop I bought in 2015 which also has an Intel iGPU, but a more recent model. So I have reason to believe the issue could only manifest on ancient 10+ year old GPUs...

Disclaimer

Yes, I know that it's absolutely horrible that I have 22 copies of the exact same (!) shader code, just with different settings... 🥶 But we have no preset support, so this was the only practical way to release my set of shaders this year (and to do this before eXoDOS v6 is out, so our shader work can be included) using our current shader pipeline which is very barebones.

I have plans for how to make this a lot nicer in the future; most likely these 22 copies will be replaced by a single shader, and I'll just send it different settings at runtime when there's a "shader change" (which is really just a "preset change"). But that's for the future... 😄

Manual testing

Tested a few fixed shaders on Burrito's box to make sure I understood the problem.

Regression tested all fixed shaders on my M1 MacBook to make sure they still work.

Checklist

Please tick the items as you have addressed them. Don't remove items; leave the ones that are not applicable unchecked.

I have:

• followed the project's contributing guidelines and code of conduct.
• performed a self-review of my code.
• commented on the particularly hard-to-understand areas of my code.
• split my work into well-defined, bisectable commits, and I named my commits well.
• applied the appropriate labels (bug, enhancement, refactoring, documentation, etc.)
• checked that all my commits can be built.
• confirmed that my code does not cause performance regressions (e.g., by running the Quake benchmark).
• added unit tests where applicable to prove the correctness of my code and to avoid future regressions.
• made corresponding changes to the documentation or the website according to the documentation guidelines.
• locally verified my website or documentation changes.

[Burrito78]

Works in windowed and fullscreen with various resolutions. Thanks!

[Grounded0]

Check at least a few modified shaders for performance regressions.

[Burrito78]

Check at least a few popular shaders for performance regressions.
I checked QTD with crt-auto, sharp and none using default cfg and got got around 137-140fps which is the highest result you're going to get on that machine. So I think we are fine.

Did check "sharp", "crt-auto" and "none" and got around 140fps which is the highest score you'll get on this machine.

[FeralChild64]

Tested on my Intel ARC, no problems found.

[kcgen]

Tested CGA, Tandy, EGA, VGA (various), and VESA modes on the Pi4 (Broadcom OpenGL 4.x compatible) and found no problems.

[johnnovak]

Check at least a few modified shaders for performance regressions.

Not possible to introduce any by commenting unused stuff out.

[johnnovak]

Thanks for the testing guys. Can someone approve this then please @kcgen @Burrito78 @FeralChild64