Sdl2InputDriver.Dispose() would call SDL_DelEventWatch with a different
"user_data" parameter than SDL_AdEventWatch. This caused the EventFilter
to remain registered and subsequently crash when closing and reopening a
window.
Added IDisposable interface to all classes holding native data that must be freed. OpenTK.Toolkit.Init() now returns an IDisposable instance that can be used to cleanup all native data held by OpenTK. This is useful when re-initializing OpenTK (possibly in a new AppDomain), as is the case in the Example browser.