Mark all functions as leaf

[Keithcat1]

Can be done with the functions > leaf config in ffigen's section of pubspec.yaml. This should improve performance for things like SDL_PollEvent which are called often, and allow passing typed lists to functions that normally have to allocate every time, such as the audio functions I think. Don't know if that feature is yet stable though. Leaf functions must not call Dart codee though, but it is possible to mark functions as not leaf specifically I think.