@kas1e
I'll be back at work tomorrow, can't test anything I'm afraid, but you are welcome to tr.
Here are the shaders (not sure if they will show anything out of the box, maybeu its better to get a daily build of scummvm, that will at least show a test screen and some of the compilation errors?).
I can give you a list of non working and buggy ones, but might also not be able to do so before the middle of February.
Anyway, just for a quick check:
https://github.com/libretro/glsl-shadersShaders that error during compile;
borders/gameboy-player/gameboy-player-tvout... (all of them)
and
bilinear
Shaders that are wrongly mirrored horizontally:
handheld/console-border/psp-2x (2x-6x)
ntsc/artifact-colors (all of them)
That are just examples, I have much more that don't work, but I believe the root is the same problem, which, once fixed, will make all others work away as well.
A working shader would be
eagle/supereagle
Thank you for taking a look.
Maybe we can make our gl port even better