Converts packed RGB to planar and back.

TBR=kjellander@chromium.org
BUG=libyuv:728
TEST=MergeRGBPlane_Opt and SplitRGBPlane_Opt unittests added

Change-Id: Ida59af940afcb1fc4a48bbf62c714f592665c3cc
Reviewed-on: https://chromium-review.googlesource.com/658069Reviewed-by: Frank Barchard <fbarchard@google.com>
Reviewed-by: Cheng Wang <wangcheng@google.com>