Add a speed feature to give the tighter partition search
range. Before partition search, calculate the histogram
of the partition sizes of the left, above and previous
co-located blocks of the current block. If the variance of
observed partition sizes is small enough, adjust the search
range around the mean partition size, which will be tigher.

The feature is currently turned on at speed 2. Experiments on
sample youtube clips show on average the runtime is reduced
by 3-7%.

For hard stdhd clips:
park_joy_1080p @ 15000kbps:       509251 ms -> 491953 ms (3.3%)
pedestrian_area_1080p @ 2000kbps: 223941 ms -> 214226 ms (4.3%)

The PSNR performance is changed:
derf: -0.112%
yt:   -0.099%
hd:   -0.090%
stdhd:-0.102%

Change-Id: Ie205ec5325bf92ec5676c243e30ba9d0adca10f2