Subsystems may be shut down while they're still being used

[daggerbot]

If two different instances of a subsystem object are acquired from the main Sdl object, the subsystem will be shut down when any one of these instances is dropped. For example:

extern crate sdl2;

fn main() {
    let sdl = sdl2::init().unwrap();
    let _video1 = sdl.video().unwrap();
    {
        let _video2 = sdl.video().unwrap();
    } // SDL_QuitSubSystem is called here
    // ...
}

SDL_QuitSubSystem is called when _video2 is dropped even though _video1 is still alive. This is because each instance holds a reference to a different SubsystemDrop.

A possible solution is to store a Weak<SubsystemDrop> for each subsystem in the main SdlDrop. Then when the subsystem is requested, the SubsystemDrop can be acquired from here if it already exists.

[Cobrand]

Like in my reply there: #1247 (comment)

We could use atomic integers to prevent a subsystem being destroyed when it shouldn't. And while we're at it, we could prevent a subsystem from being initialized if it has already been initialized.