Add joint motion search in comp_inter_inter mode(experiment)

In current code, motion vectors got from single prediction mode are used
in compound prediction mode directly. These motion vectors may not give
accurate prediction since they are searched independently. In this patch,
we took Pascal's suggestion, and did joint motion search in compound
prediction mode to find better motion vectors in this situation.
Test results:
Overall PSNR: 0.570%(derf), 0.918%(stdhd);
SSIM: 0.572%(derf), 1.009%(stdhd);

The encoder is a little slower. This can be improved since some c
code is used in motion search.

Change-Id: Ib30c9240f6c56c9b070867b4ca89412a76d9f3c6