Find the scalar the produces the maximum contrast for a signal given a
background.
alpha = displayMaxContrast(signalDir,backDir)
With the scalar alpha applied to the signal, the total signal reaches
one or the other display boundary in the equation. This routine is
useful, for example, in finding the maximum contrast cone-isolating
stimulus that can be displayed on a particular monitor.
0 <= alpha*signalDir + backDir
alpha*signalDir + backDir <= 1
Copyright ImagEval Consultants, LLC, 2005.