For speed 5, 1 pass CBR: Don't use the nonrd_pick_partition
on the segment, rather use choose_partitioning followed by
nonrd_select_partition (as is done on base segment).

Little/no quality loss on RTC and RTC_derf (< 0.3%),
speedup of at least 5%.

Change-Id: I5273d5f950e60adf5e437b4ca8c4f63964641e83