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