Fix overlay blending multiplying for transparent alpha

[itsmeow]

BYOND seems to just copy the source RGB if the destination is 0 alpha, rather than doing multiplication by the source alpha.

/world
	fps = 25
	icon_size = 32
	view = 1

/mob/verb/Y()
	set name = "Y"
	var/atom/movable/Z = new (loc)
	var/icon/A = icon('transparent.dmi', "nothing")
	A.Blend(icon('transparent.dmi', "thing"), ICON_OVERLAY)
	Z.icon = A

BYOND result:

OpenDream result (old) - see how it's desaturated/darker?

OpenDream result (new) - no longer desaturated.