When width was odd Y channel wrote an extra pixel.
This change splits the Y from UV into a temporary
buffer and memcpy's to the destination.  Performance
is slower.

Was
YUY2ToNV12_Any (307 ms)
YUY2ToNV12_Unaligned (213 ms)
TestYUY2ToNV12 (181 ms)
YUY2ToNV12_Opt (177 ms)
YUY2ToNV12_Invert (177 ms)

Npw
YUY2ToNV12_Any (300 ms)
YUY2ToNV12_Unaligned (226 ms)
YUY2ToNV12_Invert (206 ms)
TestYUY2ToNV12 (184 ms)
YUY2ToNV12_Opt (181 ms)
TBR=harryjin@google.com
BUG=libyuv:545

Review URL: https://codereview.chromium.org/1593833002 .