The concept:

There's too much noise in source pixels for variance and at low bitrate
the reconstructed looks nothing like the source so we have problems
getting good partitionings with either.   This skirts the issue by using
a box blur scaled down version for variance calculations.  To compare
against source_var_ moved keyframe to be rd based like source_var.

Change-Id: Ie3babdbfadae324b7b5a76bea192893af27f0624