oggtheora: Fix initial pts

code based on the solution in vorbis
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/oggparsetheora.c

@@ -151,9 +151,51 @@ theora_gptopts(AVFormatContext *ctx, int idx, uint64_t gp, int64_t *dts)
     return iframe + pframe;
 }
 
+static int theora_packet(AVFormatContext *s, int idx)
+{
+    struct ogg *ogg = s->priv_data;
+    struct ogg_stream *os = ogg->streams + idx;
+    int duration;
+
+    /* first packet handling
+       here we parse the duration of each packet in the first page and compare
+       the total duration to the page granule to find the encoder delay and
+       set the first timestamp */
+
+    if (!os->lastpts) {
+        int seg;
+        uint8_t *last_pkt = os->buf + os->pstart;
+        uint8_t *next_pkt = last_pkt;
+        int first_duration = 0;
+
+        duration = 0;
+        for (seg = 0; seg < os->nsegs; seg++) {
+            if (os->segments[seg] < 255) {
+                if (!duration)
+                    first_duration = 1;
+                duration++;
+                last_pkt = next_pkt + os->segments[seg];
+            }
+            next_pkt += os->segments[seg];
+        }
+        os->lastpts = os->lastdts   = theora_gptopts(s, idx, os->granule, NULL) - duration;
+        s->streams[idx]->start_time = os->lastpts + first_duration;
+        if (s->streams[idx]->duration)
+            s->streams[idx]->duration -= s->streams[idx]->start_time;
+    }
+
+    /* parse packet duration */
+    if (os->psize > 0) {
+        os->pduration = 1;
+    }
+
+    return 0;
+}
+
 const struct ogg_codec ff_theora_codec = {
     .magic = "\200theora",
     .magicsize = 7,
     .header = theora_header,
+    .packet = theora_packet,
     .gptopts = theora_gptopts
 };