Button positions for add_button, offsets from the bottom right

Property definitions

gamnit $ VirtualGamepad :: _button_positions
	# Button positions for `add_button`, offsets from the bottom right
	private var button_positions = new Array[Point[Float]].with_items(
		new Point[Float](-150.0, 150.0),
		new Point[Float](-150.0, 350.0),
		new Point[Float](-150.0, 550.0),
		new Point[Float](-350.0, 150.0),
		new Point[Float](-350.0, 350.0),
		new Point[Float](-350.0, 550.0))
lib/gamnit/virtual_gamepad/virtual_gamepad.nit:131,2--138,34