Revert "Move compound segment mask buffer from mbmi to xd" This reverts commit 2cf73eb84e1d324c69e9b4a118621a274e761123. BUG=https://bugs.chromium.org/p/aomedia/issues/detail?id=503 Change-Id: I2b3a4a77854044cb7951c8e64961e72255dfea85