Adding support for muxing VP8 Alpha files

This patch adds support for muxing VP8 Alpha Files. The Alpha channel data is
placed in BlockAdditional element of the matroska container. More information
& exact spec on how this is implemented can be found here: http://goo.gl/wCP1ySigned-off-by: Vignesh Venkatasubramanian <vigneshv@google.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/matroskaenc.c

@@ -662,6 +662,12 @@ static int mkv_write_tracks(AVFormatContext *s)
                         put_ebml_uint(pb, MATROSKA_ID_VIDEOSTEREOMODE, st_mode);
                 }
 
+                if ((tag = av_dict_get(st->metadata, "alpha_mode", NULL, 0)) ||
+                    (tag = av_dict_get( s->metadata, "alpha_mode", NULL, 0)) ||
+                    (codec->pix_fmt == AV_PIX_FMT_YUVA420P)) {
+                    put_ebml_uint(pb, MATROSKA_ID_VIDEOALPHAMODE, 1);
+                }
+
                 if (st->sample_aspect_ratio.num) {
                     int64_t d_width = av_rescale(codec->width, st->sample_aspect_ratio.num, st->sample_aspect_ratio.den);
                     if (d_width > INT_MAX) {
@@ -1136,9 +1142,10 @@ static void mkv_write_block(AVFormatContext *s, AVIOContext *pb,
 {
     MatroskaMuxContext *mkv = s->priv_data;
     AVCodecContext *codec = s->streams[pkt->stream_index]->codec;
-    uint8_t *data = NULL;
-    int offset = 0, size = pkt->size;
+    uint8_t *data = NULL, *side_data = NULL;
+    int offset = 0, size = pkt->size, side_data_size = 0;
     int64_t ts = mkv->tracks[pkt->stream_index].write_dts ? pkt->dts : pkt->pts;
+    uint64_t additional_id = 0;
 
     av_log(s, AV_LOG_DEBUG, "Writing block at offset %" PRIu64 ", size %d, "
            "pts %" PRId64 ", dts %" PRId64 ", duration %d, flags %d\n",
@@ -1156,14 +1163,46 @@ static void mkv_write_block(AVFormatContext *s, AVIOContext *pb,
         offset = 8;
     }
 
-    put_ebml_id(pb, blockid);
-    put_ebml_num(pb, size+4, 0);
-    avio_w8(pb, 0x80 | (pkt->stream_index + 1));     // this assumes stream_index is less than 126
-    avio_wb16(pb, ts - mkv->cluster_pts);
-    avio_w8(pb, flags);
-    avio_write(pb, data + offset, size);
-    if (data != pkt->data)
-        av_free(data);
+    side_data = av_packet_get_side_data(pkt,
+                                        AV_PKT_DATA_MATROSKA_BLOCKADDITIONAL,
+                                        &side_data_size);
+    if (side_data) {
+        additional_id = AV_RB64(side_data);
+        side_data += 8;
+        side_data_size -= 8;
+    }
+
+    if (side_data_size && additional_id == 1) {
+        ebml_master block_group, block_additions, block_more;
+        block_group = start_ebml_master(pb, MATROSKA_ID_BLOCKGROUP, 0);
+        put_ebml_id(pb, MATROSKA_ID_BLOCK);
+        put_ebml_num(pb, size+4, 0);
+        avio_w8(pb, 0x80 | (pkt->stream_index + 1)); // track number
+        avio_wb16(pb, ts - mkv->cluster_pts);
+        avio_w8(pb, flags);
+        avio_write(pb, data + offset, size);
+        if (data != pkt->data)
+            av_free(data);
+        block_additions = start_ebml_master(pb, MATROSKA_ID_BLOCKADDITIONS, 0);
+        block_more = start_ebml_master(pb, MATROSKA_ID_BLOCKMORE, 0);
+        put_ebml_uint(pb, MATROSKA_ID_BLOCKADDID, 1);
+        put_ebml_id(pb, MATROSKA_ID_BLOCKADDITIONAL);
+        put_ebml_num(pb, side_data_size, 0);
+        avio_write(pb, side_data, side_data_size);
+        end_ebml_master(pb, block_more);
+        end_ebml_master(pb, block_additions);
+        end_ebml_master(pb, block_group);
+    }
+    else {
+        put_ebml_id(pb, blockid);
+        put_ebml_num(pb, size+4, 0);
+        avio_w8(pb, 0x80 | (pkt->stream_index + 1));     // this assumes stream_index is less than 126
+        avio_wb16(pb, ts - mkv->cluster_pts);
+        avio_w8(pb, flags);
+        avio_write(pb, data + offset, size);
+        if (data != pkt->data)
+            av_free(data);
+    }
 }
 
 static int srt_get_duration(uint8_t **buf)