Set draw order, see draw_order

Property definitions

gamnit $ Sprite :: draw_order=
	# Set draw order,  see `draw_order`
	fun draw_order=(value: Int)
	do
		if isset _draw_order and value != draw_order then needs_remap
		draw_order_direct = value
	end
lib/gamnit/flat/flat_core.nit:263,2--268,4