Collect only relative mouse events and hide cursor

Check relative_mouse_mode to see if the mode could be applied, if not, see error.

Property definitions

sdl2 $ SDL :: relative_mouse_mode=
	# Collect only relative mouse events and hide cursor
	#
	# Check `relative_mouse_mode` to see if the mode could be applied,
	# if not, see `error`.
	fun relative_mouse_mode=(value: Bool) `{
		SDL_SetRelativeMouseMode(value);
	`}
lib/sdl2/sdl2_base.nit:86,2--92,3