Scale Down by 2 use ld2 and urhadd
Scale Down by 2 use ld2 and urhadd

urhadd is a rounded average.  Linear filter wants to average
horizontally, so use ld2 to separate even and odd pixels.

TBR=jkellander@chromium.org
BUG=None
TEST=LibYUVScaleTest.*ScaleDownBy2*

Change-Id: Id667288a030e72ce8e1c1d6719b69c555c0db063
Reviewed-on: https://chromium-review.googlesource.com/642448
Commit-Queue: Frank Barchard <52415189a3f67d375195cbbb6fdb1f1759498e57@google.com>
Reviewed-by: Cheng Wang <92a840dd84c11ec2f12edfeb28e694a62b5c28e2@google.com>

lemenkov/libyuv,lemenkov/libyuv,lemenkov/libyuv,lemenkov/libyuv