dca: set AVCodecContext frame_size for DTS audio

Set the frame size when decoding DTS audio.

This has the side effect of fixing the computation of timestamps for DTS-HD in compute_pkt_fields.  Since frame_size is
not currently set, the duration of a frame is being guessed based on the streams bitrate.  But for DTS-HD, the bitrate
currently used is the rate of the DTS core which is much different than the whole DTS-HD stream and leads to a wildly
inaccurate frame duration estimate.
Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>

libavcodec/dca.c

@@ -1650,6 +1650,7 @@ static int dca_decode_frame(AVCodecContext * avctx,
     //set AVCodec values with parsed data
     avctx->sample_rate = s->sample_rate;
     avctx->bit_rate = s->bit_rate;
+    avctx->frame_size = s->sample_blocks * 32;
 
     s->profile = FF_PROFILE_DTS;