ENH: Add MarkDuplicateFramesInvalidImageFilter.

The screengrabber's frame rate is often higher that the ultrasounds's frame
rate, so detect duplicate frames (or partial duplicates with an incomplete
refresh) by subtracting subsequent frames.
ENH: Add duplicate frame detection functionality to ConvertInnerOpticToPlus.
Merge pull request #159 from thewtex/detect-duplicate-frames

Detect duplicate frames