Add prune_ref_frame_for_rect_partitions feature

Add prune_ref_frame_for_rect_partitions feature

Add a speed feature to prune reference frames for rectangular
partitions. Rectangular partition RD search happens after square
partition RD search. With this feature, we keep record of the ref
frames picked by square partitions, and only consider those ref
frames during rect partition RD search.

With this feature on, the computation cost of rect partition RD
search is greatly reduced, so we can afford to skip rect partition
RD search less aggressively.

Overall, both compression and encoding speed are improved. Only
speed 0 is affected.

Coding gains:
              lowres    midres    hdres
ovr psnr      0.00%    -0.36%    -0.37%
avg psnr      0.00%    -0.36%    -0.36%

Tested encoding speed with QP=40 on about 30 sequences.
Speed gains:
              lowres    midres    hdres
average       13.4%      7.1%     6.1%
max           28.0%     12.0%     9.8%

Change-Id: Id5f36dd2ac75028ae98550d67b0a524aa251b692