This allows a mask for mrc-tx to be sent in the bitstream for
inter or intra 32x32 transform blocks. The option to send the mask
vs build it from the prediction signal is currently controlled with
a macro. In the future, it is likely the macro will be removed and it
will be possible for a block to select either method. The mask building
functions are still placeholders and will be filled in in a followup.

Change-Id: Ie27643ff172cc2b1a9b389fd503fe6bf7c9e21e3