A modified canvas which clips all drawing commands.

	initialization
	canvas:clipRect:

	private
	apply:

	canvas methods	top
	clipRect Return the currently active clipping rectangle
	contentsOfArea:into: Return the contents of the given area
	form
	isBalloonCanvas
	isShadowDrawing
	shadowColor Return the current override color or nil if no such color exists

	initialization	top
	canvas:clipRect:

	private	top
	apply: apply the given block to the inner canvas with clipRect as the clipping rectangle

	instance creation	top
	canvas:clipRect: