Modified key frame detection.

Modified key frame detection.

Address poor key frame detection in some content.

This patch improves on poor key frame / scene cut detection observed
with some test content. The content in question was letter boxed film
style material and also had quite low contrast. For both 1080P and 4K
multiple genuine scene cuts were being missed.

The changes alter the conditions for marking a transition as a "flash" rather
than a scene change. The new code still deals well with genuine flashes as
observed in the "crew" test clip, without falsely flagging some of the
the scene cuts in the "film" test clip.

The new film test clip also had some "flash" frames caused by a lightning
effect and in one case a flash occurred right before a scene change. This
caused a misplacement of the key frame but has been addressed by a new
clause that requires the coded error for the next frame after a candidate
key frame to be lower than the current frame.

The patch also changes the way in which neutral blocks (similar inter and
inter error) are handled in the candidate key frame decision in a way which
hopefully handles the letter boxed format better.

During wider testing some film clips still had missed key frames but this
patch does improve things. In the case of the initial test clip the encoder
correctly marks all 3 scene cuts vs 0 before the patch.

Testing with our standard (mainly short single kf) derf and NF test clips
is neutral.

Change-Id: I3b7dcfe7b2fb13fd0816ea46acc3e69c8bc581b3