Investigate adding overlay image to preview mode

[coleslaw481]

To help a user properly place their camera in front of their tv it would be helpful to overlay on the Pi preview mode the fringe pixels that will be used to get data from the image. This would also let the user align the led light strip with the tv.

[coleslaw481]

Prototype implementation that shows colored filled squares where the pixels the pi are examining are located. The color of the squares is the color seen by the pi camera.