Add CONVOLVE_POST_ROUNDING flag

By turning on CONVOLVE_POST_ROUNDING, in the compound inter
prediction mode, FILTER_BITS rounding is moved after the summation
of two predictions.

Note that the post rounding is only applied on non-sub8x8 block

       PSNR     BDRate
lowres -0.808%  -0.673%

Change-Id: Ib91304e6122c24d832a582ab9f5757d33eac876c