ImGuiIO::AddInputCharactersUTF8(utf8str), use it in SDL2 example

Now Unicode text input works in this example.

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.

Examples: Fix SDL2 example for Linux.

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.

Thanks Coverity

ImFontAtlas: new AddFont() API, oversampling, subpositiong, merging fonts, etc. (#182, #220, #232, #242)