Simplify Callback Setup #82

How callbacks work is very verbose and ugly. Instead, a wrapped function that sets up zig-like callbacks should be used similar to https://github.com/castholm/zig-examples/blob/master/breakout/main.zig#L943.

Ideally there would be no more ugly disable main hack too, and using callbacks would feel like a proper extension rather than have so much overhead. Having so much overhead is very discouraging for beginners, and switching from a main function to callbacks should be not too much effort if this is implemented.

I did a new approach that works well, but I won't close this until you can avoid doing this:

// Use main callbacks.comptime{_=sdl3.main_callbacks;}

I am not sure how to get SDL to export the functions for when you have the callbacks enabled but not anything else.