Add the flag to request a window using the system High-DPI mode

Property definitions

sdl2 $ SDLWindowFlags :: allow_highdpi
	# Add the flag to request a window using the system High-DPI mode
	fun allow_highdpi: SDLWindowFlags `{
		#if SDL_VERSION_ATLEAST(2, 0, 2)
			return self | SDL_WINDOW_ALLOW_HIGHDPI;
		#else
			return self;
		#endif
	`}
lib/sdl2/sdl2_base.nit:200,2--207,3