Add support for retroarch shaders

I've been trying to setup a crt scanline filter in some of the retroarch emulators. Unfortunately it seems the shaders seem to be missing. I've tried copying them over from a backup of a retropie install I had (which ran on the same hardware), but I haven't been able to get them to work. I'm not sure if that's due me not having the directories for the shaders configured properly, or if they need to compiled or something like that.

Anways, long story short. Support for the shaders would be nice, since that allows you to really improve the look of emulated games on modern screens.