Enable rectangular partition search for speed 1

Enable rectangular partition search for speed 1

This patch enables rectangular partition search on speed 1. The encoding
speed loss is reduced thanks to recently added speed features.

This only affects speed 1 low bit-depth encoding.

Coding gains:
           avg_psnr     ovr_psnr    ssim
lowres      0.577%       0.621%    0.665%
midres      1.147%       1.215%    1.148%
hdres       0.758%       0.790%    0.769%

Tested encoding speed on 15 midres and 15 hdres clips, average speed
loss:
           QP=30       QP=40        QP=50
midres     4.43%       3.72%       -1.05%
hdres      4.41%       5.65%        3.77%

Change-Id: Ifc0712becccc69f7498796359ff12dbfa63fd7b3