retroarch-cg2glsl converts Cg shaders to GLSL shaders. It can convert single shaders as well as whole folder structures in batch. 100% compatibility is not guaranteed, but almost all shaders should work fine. Cg presets (.cgp) are not converted at the moment, but converting them is as simple as copying over the .cgp, rename it to .glslp and replace references to .cg files to .glsl.
Requires Python 3 and cgc (nvidia-cg-toolkit) 3.1.