Changes to calculation of  sr_coded_error to include 0,0 case.

Experimental use of sr_coded_error in calculating correction factor
for estimating the allowable Q range.

Reinstated some code needed for calculating section_intra_rating.

Add flash detection in calculation of KF boost

Increased tolerance in testing candidate key frames (needed with
longer motion search as this tends to slightly increase inter %.

Zbin changes for 8x8.

Other minor adjustments, refactoring and bug fixes.

Reinstated some motion break out clauses in boost loop
as their removal hurt a few 50fps  clips badly in the std set.
It may be possible to remove them again later if a better way
can be found of preventing overly long gf intervals.

Change-Id: Iee686d0c31072828bb1ccd2bc63f5f1c7c548ea2