Due to the discontinuation of CG, BGFX could be an open source alternative.
The benefit of BGFX is the cross-platform support.
Just to bring up a topic concerning the future of retroarch shaders. 
Yeah, I was just looking at MAME’s commits related to this. It looks like a pretty solid option, though someone else would have to be willing to write the context and shader backend, as maister’s already pretty committed to Vulkan/SPIR-V.
Same for me. I immediately thought this could be also relevant for libretro, after I read the news about mame. Maybe it makes more sense to switch to a open source solution, because this can be further developed. The mame team already started to port the xbr shaders. In general it would make more sense if the majority of developers could agree on one shader language. Looking into that overview is already surprising : http://emulation-general.wikia.com/wiki/Shaders_and_Filters
Maister already has a new shader spec lined up that will work across both GL and Vulkan. That is good enough for us.
Bgfx is not necessary for RetroArch, and we don’t need it for a shader spec either. I will never adopt it. I don’t want all sorts of wrappers and obfuscations around GLSL/SPIR-V shader code, only the bare minimum of what is necessary.