[mythtv] [PATCH] crop/quickdnr enhancements + bugfixes

[D Banerjee]

Crop was broken, and full of ugly code - fixed.

Quickdnr:
1. I added a second threshold to quickdnr, and I can't seem to measure a
speed difference outside the margin of error, I assume this is because the
memory latency is orders of magnitude higher than the algorithm execution
time, but better for us.

It's much higher quality, and it even without a visual difference it will be
better for the encoder = lower bitrate needed.

2. Made double threshold default, single optional, and optional scaled
quality setting for double (one 0-255 integer instead of 4 pixel
thresholds).

3. Toned down filter thresholds so it will be acceptable for a much wide
range of video/users.

3. Tweaked prefetch, a few percent faster total.

-Debabrata