Add RGB565 support

Tested on the title screens of Super Mario Land 3 and Zelda Oracle of Seasons. (Also includes a variable rename and a fix for a C source file that got marked executable.) It's inside some #ifdefs that are only enabled for Libretro, so it shouldn't break the other frontends.
I didn't check if this mode renders identically, but it looks the same to me, at least. It looks like there's a bit of attempts at emulating an original GBC screen in libgambatte/src/video.cpp line 33 that I'm not sure if we want (should probably be in a shader), but I'll leave that decision to you guys; I'm happy either way.