add input descriptors

[hunterk]

enables remapping in RetroArch

[icculus]

Should we do this every frame, or is this better done in retro_set_environment?

[hizzlekizzle]

That's a good point. I stuck it in with the other input stuff, but there's indeed no reason to do it every frame. How's this look instead?

[icculus]

Looks good, thanks!