Use SDLmain library

[laanwj]

Currently the windows build is broken because of a missing WinMain function. Instead of writing custom main functionality for every platform we'd be better off to simply use the auxiliary library SDL2main and rename main to SDL_main.

Here's an example here of using SDL2main with meson: https://github.com/jpakkane/sdl2meson/blob/master/meson.build