Tokenize and write mrc mask

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
diff --git a/aom_dsp/txfm_common.h b/aom_dsp/txfm_common.h
index fa96aca..c33e38e 100644
--- a/aom_dsp/txfm_common.h
+++ b/aom_dsp/txfm_common.h
@@ -36,6 +36,7 @@
 #endif
 #if CONFIG_MRC_TX
   int *valid_mask;
+  uint8_t *mask;
 #endif  // CONFIG_MRC_TX
 #endif  // CONFIG_MRC_TX || CONFIG_LGT
 // for inverse transforms only