Merge "Initial version of alpha channel support" into experimental

diff --cc configure
Simple merge

diff --cc vp9/common/vp9_alloccommon.c
Simple merge

diff --cc vp9/common/vp9_blockd.h
Simple merge

diff --cc vp9/common/vp9_onyxc_int.h
Simple merge

diff --cc vp9/encoder/vp9_encodeframe.c
Simple merge

diff --cc vp9/encoder/vp9_encodemb.c
Simple merge

diff --cc vp9/encoder/vp9_onyx_if.c
Simple merge

diff --cc vp9/encoder/vp9_onyx_int.h
Simple merge

diff --cc vp9/encoder/vp9_rdopt.c
Simple merge