Adds code to detect dead zone bars at the top and bottom
of reformatted letterbox video (note that the code only
looks at the top of the image and assumes any dead zone
is symmetrical).  Use of this to adapt rate control etc.
will follow in a subsequent patch.

Also counts other blocks (excluding the dead zone) that
have no intra signal. The presence of a significant
number of such blocks can be used as a identify that the frame
may be artificial (e.g.  animation, screen capture, graphics).
This patch contains plumbing only and does not use
the signal.

Change-Id: I59bc93529cd4065416cef773e405fda3ae006a20