It'll convert the utf8 string to ImWchar's and passes each of them
to AddInputCharacter().
Very handy for SDL2 SDL_TEXTINPUT events, which provide a buffer with an
UTF-8 string.
The header is called "SDL_opengl.h" - case matters on Linux.
The main() function must indeed be called main(), on Windows it'll be
renamed to SDL_main (for SDL2main.lib) by #define in SDL_main.h
(which is included by SDL.h)
I added an entry to the example's README that describes how to build
it on Linux.