Fix for ScaleDownBy4_Linear_16
Fix for ScaleDownBy4_Linear_16

The unittest compares the results of 8 and 16 bit scaling
and expects them to be the same.  This CL makes the 16 bit
scaling filter logic match.

Bug: libyuv:749
Test: LibYUVScaleTest.DISABLED_ScaleDownBy4_Linear_16
Change-Id: Ifb3ca4d770ef38f9f16abe9b9aeb843b779bf371
Reviewed-on: https://chromium-review.googlesource.com/772370
Reviewed-by: Weiyong Yao <a5c97fa71e6dd5ce51416b57777539e11f5828d8@chromium.org>
Commit-Queue: Frank Barchard <52415189a3f67d375195cbbb6fdb1f1759498e57@google.com>

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