Add rounding option into av1_convolve

Use a round flag in ConvolveParams to indicate if the destination buffer
has the result rounded by FILTER_BITS or not.
This CL is part of the goal of reducing interpolation rounding error in
compound prediction mode.

Change-Id: I49e522a89a67a771f5a6e7fbbc609e97923aecb6
7 files changed