mythcommflag: add CommDetectLogoBorder
mythcommflag: add minPixelsInMask to ClassicLogoDetector.

Back in 2005, a threshold of 50 minimum pixelsInMask was established.
I don't know whether that was tested against SD or HD resolutions.
I do know that in 2010, mythcommflag was changed to use ffmpeg's
lowres support, effectively dividing the video area by 16.
But the 50 pixel minimum was not adjusted accordingly.
I believe the minimum threshold should vary with the video's area.
I am using 1280x720 (for 720p) video as the baseline.
This should improve logo detection for SD video.