In preparation for Daala unified LBD/HBD TX, add (and use) is_hbd
field in TxfmPama structure.  This field indicates whether or not
pixel data is using 8 or 16 bit reference buffers (currently ambiguous
in the case of 8 bit input).

Change-Id: I28bca792a48ffa00e208617adb072b08ff816e3c