Handle scale down by factor of 2 efficiently by calling SIMD for multiple of 16 destination pixels, and C for remainder.
BUG=314
TESTED=out\release\libyuv_unittest.exe --gtest_catch_exceptions=0 --gtest_filter=*.ScaleDownBy2*
R=bcornell@google.com

Review URL: https://webrtc-codereview.appspot.com/48689004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@1344 16f28f9a-4ce2-e073-06de-1de4eb20be90