Its prototype declared at the top of the translation unit contains the
static qualifier, so the function itself should also contain it to make
it a proper internally linked function.
The deleter can just be set in the constructor and maintained throughout
the lifetime of the object.
If a contained pointer is null, then the deleter won't execute, so this
is safe to do. We don't need to swap it out with a version of a deleter
that does nothing.
Since C++17, the introduction of deduction guides for locking facilities
means that we no longer need to hardcode the mutex type into the locks
themselves, making it easier to switch mutex types, should it ever be
necessary in the future.
Any SDL invocation can call the even callback on the same thread, which can call GetSDLJoystickBySDLID and eventually cause double lock on joystick_map_mutex. To avoid this, lock guard should be placed as closer as possible to the object accessing code, so that any SDL invocation is with the mutex unlocked
Changes the interface as well to remove any unique methods that
frontends needed to call such as StartJoystickEventHandler by
conditionally starting the polling thread only if the frontend hasn't
started it already. Additionally, moves all global state into a single
SDLState class in order to guarantee that the destructors are called in
the proper order
