libavformat: backport upstream fix for DTS-HD passthrough

"Align IEC 61937 length_code for DTS-HD so that
(length_code & 0xf) == 0x8. This is reportedly needed with some
receivers."

SHA1: ab9c00b

Thanks to Anssi Hannula for the fix and pointing it out to me.

mythtv/external/FFmpeg/libavformat/spdifenc.c

@@ -214,7 +214,10 @@ static int spdif_header_dts4(AVFormatContext *s, AVPacket *pkt, int core_size,
     }
 
     ctx->out_bytes   = sizeof(dtshd_start_code) + 2 + pkt_size;
-    ctx->length_code = ctx->out_bytes;
+
+    /* Align so that (length_code & 0xf) == 0x8. This is reportedly needed
+     * with some receivers, but the exact requirement is unconfirmed. */
+    ctx->length_code = FFALIGN(ctx->out_bytes + 0x8, 0x10) - 0x8;
 
     av_fast_malloc(&ctx->hd_buf, &ctx->hd_buf_size, ctx->out_bytes);
     if (!ctx->hd_buf)