It's kind of a bummer that L/R don't actually do anything... we should add ZL/ZR support at some point.
Also note that GameCube binds X to 'back' rather than B, this will be fixed by SDL_ActionSet for 2.5.
They are not used yet in this commit - this just adds the graphics and
data for the glyphs. It also adds a <fallback> tag to font.fontmeta to
use buttons_8x8.
The icons themselves are made by Reese Rivers - see #859.