This patch simplifies the setting and use of the do_post_rounding
flag consistently across all convolve/warp operations. It is always
set now to 1 for compound modes, but the masked blending operation
can subsequently turn it off.
To accomplish this, for the warped case, the use of the conv_params->dst
buffer is made consistent with the other non-warp convolves,
in the sense that it is used only when is_compound is 1.

Change-Id: If3a37ffa65c3ca75f1df66ca427e6b5df86ac72f