When scaling down by 2 the formula should round consistently.
(a+b+c+d+2)/4
The C version did but the SSE2 version was doing 2 averages.
avg(avg(a,b),avg(c,d))
This change uses a sum, then rounds.

R=dhrosa@google.com, harryjin@google.com
BUG=libyuv:447,libyuv:527

Review URL: https://codereview.chromium.org/1513183004 .