Several improvements we made in good-quality mode can be added
into real-time mode to speed up encoding in speed 1, 2, and 3
with small quality loss. Tests using tulip clip showed:

--rt --cpu-used=-1
(before change)
PSNR: 38.028
time: 1m33.195s
(after change)
PSNR: 38.014
time: 1m20.851s

--rt --cpu-used=-2
(before change)
PSNR: 37.773
time: 0m57.650s
(after change)
PSNR: 37.759
time: 0m54.594s

--rt --cpu-used=-3
(before change)
PSNR: 37.392
time: 0m42.865s
(after change)
PSNR: 37.375
time: 0m41.949s

Change-Id: I76ab2a38d72bc5efc91f6fe20d332c472f6510c9