BalloonEdgeData defines an entry in the internal edge table of the Balloon engine.

Instance Variables:
index <Integer> The index into the external objects array of the associated graphics engine
xValue <Integer> The computed x-value of the requested operation
yValue <Integer> The y-value for the requested operation
height <Integer> The (remaining) height of the edge
source <Object> The object from the external objects array

	computing
	stepToFirstScanLine stepToNextScanLine

	accessing	top
	index
	index:
	lines
	lines:
	source
	source:
	xValue
	xValue:
	yValue
	yValue:
	zValue
	zValue:

	computing	top
	stepToFirstScanLine
	stepToNextScanLine