No separate Factory.Embedded for SDL2

Sdl2Factory supports both desktop and embedded profiles, there is no
need to construct a separate Embedded profile.
Implemented IDisposable in Sdl2Joystick

Sdl2Joystick calls SDL_JoystickOpen, which means we must call
SDL_JoystickClose on shutdown.
Moved all input processing to Sdl2InputDriver

This reduces the duplication of code between Sdl2NativeWindow and
Sdl2InputDriver. Sdl2InputDriver is now solely responsible for handling
input.
Fixed file not found error

The debug.log would be deleted after launching an example, causing the
example browser to complain that the file cannot be found. In reality,
it should be deleted before launching an example - fixed.