Most existing first pass stats are stored in a form normalized to a
macro-block scale. However the error scores for intra / inter etc were
stored as frame level values but mainly used as MB level values.

This change  fixes that. Normalized per MB values make comparisons
between different formats easier and in any case this is usually what is
wanted.

An change in results should be limited to slight differences in rounding.

*** Change after patch 8 +2 requiring new approval.

Final pre-submit testing showed  one 4K clip with above expected change.
Investigation showed this was due to a value used to test for ultra low intra
complexity in key frame detection. This was a per frame not per MB value but
also did not scale with frame size. Replacement with a small per MB value
(based on original per frame value and cif frame size) resolved the KF detection
problem.

Also converted kf_group_error_left to a double in line with other error values
to reduce rounding problems in KF group bit allocation

All clips and sets now show nominal (or 0) change as expected.

Change-Id: Ic2d57980398c99ade2b7380e3e6ca6b32186901f