sdl2::Event::ControllerDeviceAdded could be made friendlier

[ticky]

After a long time trying to figure this out, I’ve discovered that SDL2 is using a signed integer for its joystick IDs so it can be both compared to, and used as an error code by SDL_NumJoysticks.

This means that all ControllerDevice events return which as an sint32, while only ControllerDeviceAdded actually intends to be used as a signed integer. Meanwhile, the ControllerAxis and ControllerButton events use the SDL_JoystickID, which is a uint underneath.

Given that the intent is for the “which” returned by ControllerDeviceAdded to be used in SDL_GameControllerOpen (as it is impossible to use it without first opening it!), it currently has an incompatible type with sdl::GameControllerSubsystem#open, where we’re accepting only a u32.

Given that rust-sdl2 converts other SDL2 errors to native Rust errors, and sdl2::GameControllerSubsystem#num_joysticks already does, I'd propose that ControllerDeviceAdded events be updated to supply which as a u32, so it can be used directly with open without requiring a confusing type conversion!