Retropie doesn't install slang shaders

[barbudreadmon]

See https://retropie.org.uk/forum/topic/31557/shaders-don-t-appear-to-be-selectable-only-when-using-lr-beetle-psx

Depending on the platform & core, sometimes RetroArch will only accept glcore & vulkan as video drivers, which require the usage of slang shaders.
Apparently, shaders in Retropie are installed from the hard fork https://github.com/RetroPie/common-shaders, which doesn't include slang shaders and also seems to use fairly old versions of glsl shaders.

[joolswills]

See #3395