This is pretty insane since SDL2 completely ignores the keysym value
of the X11 keyboard event, so here's what we do...

1) When there's a keypress that includes right-alt, store the sym/mod
2) When we get text input, if it's the same as we would get if ALT
   wasn't pressed for the last keydown, parse through the mapping
   (ie: handle ALT keys)
3) If it's different, use that (gets AltGr modified value)

I get that AltGr is hard in a cross-platform way, but just pretending
it doesn't exist at all for key input is a very weird choice.