ScreeningMorph uses its first submorph as a screen, and its second submorph as a source. It also wants you to choose (when showing only the screen) the passing color in the screen. It then makes up a 1-bit mask which clips the source, and displays transparently outside it.

	geometry
	containsPoint: layoutChanged

	initialization
	addMorph: initialize privateRemoveMorph:

	menu
	addCustomMenuItems:hand: chooseBlockingColor choosePassingColor exchange showScreenOnly showScreenOverSource showScreened showSourceOnly

	private
	screenForm screenMorph sourceMorph

	drawing	top
	fullDrawOn: Draw the full Morphic structure on the given Canvas

	geometry	top
	containsPoint:
	layoutChanged

	initialization	top
	addMorph:
	initialize
	privateRemoveMorph: Private! Should only be used by methods that maintain the ower/submorph invariant.

	menu	top
	addCustomMenuItems:hand: Add morph-specific items to the given menu which was invoked by the given hand. This method provides is invoked both from the halo-menu and from the control-menu regimes.
	chooseBlockingColor
	choosePassingColor
	exchange
	showScreenOnly
	showScreenOverSource
	showScreened
	showSourceOnly

	private	top
	screenForm
	screenMorph
	sourceMorph