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/772370Reviewed-by: Weiyong Yao <braveyao@chromium.org>
Commit-Queue: Frank Barchard <fbarchard@google.com>