postproc: Tweaks to line drawing and blending.

Turned down the blending level to make colored blocks obscure
the video less.
Not blending the entire block to give distinction to macro
block edges.
Added configuration so that macro block blending function can
be optimized.
Change to constrain line as to when dx and dy are computed.
Now draw two lines to form an arrow.

Change-Id: I986784e6abff65ea3e0d1437dfca7d06d44ede71