Low level support for 12 bit 420, 422 and 444 YUV video frame conversion.

BUG=libyuv:560, chromium:445071
TEST=LibYUVPlanarTest.TestHalfFloatPlane on windows
R=hubbe@chromium.org, wangcheng@google.com

Review URL: https://codereview.chromium.org/2387713002 .