Further firstpass.c changes.

base the static image test off a measure of 0,0 motion
instead of the decay accumulator value.

Change "transition to still detection" to compare the
decay rate from successive frames.

Minor tweak to the arf extra boost given based on the
number of frames affected.

Removed unused variable mod_err_per_mb_accumulator.

Change-Id: Idd8360083ad409e45f133ce97dd2488259003e64