Added a speed feature use_square_partition_only

This commit adds a speed feature where only squared partition are
evaluated in partition picking. Enable this feature in cpu-used 2
reduces encoding time by ~30%.

loss of compression:
-0.9% on cif set
-1.23% on stdhd

Change-Id: Ia6fad11210f0b78365abb889f9245604513be5b9