Migrated from "libsdl:2" to "libsdl2".

Migrated from "sdl-image:2" and "sdl-ttf:2" to "sdl2-image" and "sdl2-ttf" as
well. To avoid merge conflicts, removed the former ebuilds from the overlay.
This should repair the brunt of the SDL2 damage in perpetuity, for which I am
erstwhile grateful.