mlpdec: support TrueHD streams with an Atmos substream

The fourth substream is being discarded, since its not raw audio data,
but an encoded Atmos stream which needs a specialized decoder.

Fixes decoding of the true hd stream from Transformers\ -\ Age\ of\ Extinction\ 2014\ 1080P-003.mkv

Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavcodec/mlp.h

@@ -45,7 +45,7 @@
 /** Maximum number of substreams that can be decoded.
  *  MLP's limit is 2. TrueHD supports at least up to 3.
  */
-#define MAX_SUBSTREAMS      3
+#define MAX_SUBSTREAMS      4
 
 /** which multiple of 48000 the maximum sample rate is */
 #define MAX_RATEFACTOR      4

libavcodec/mlpdec.c

@@ -355,7 +355,9 @@ static int read_major_sync(MLPDecodeContext *m, GetBitContext *gb)
     m->access_unit_size_pow2 = mh.access_unit_size_pow2;
 
     m->num_substreams        = mh.num_substreams;
-    m->max_decoded_substream = m->num_substreams - 1;
+
+    /* limit to decoding 3 substreams, as the 4th is used by Dolby Atmos for non-audio data */
+    m->max_decoded_substream = FFMIN(m->num_substreams - 1, 2);
 
     m->avctx->sample_rate    = mh.group1_samplerate;
     m->avctx->frame_size     = mh.access_unit_size;