avformat/oggparsevp8: fix pts calculation on pages ending with an invisible frame

Signed-off-by: James Almer <jamrial@gmail.com>

libavformat/oggparsevp8.c

@@ -82,7 +82,11 @@ static uint64_t vp8_gptopts(AVFormatContext *s, int idx,
     struct ogg *ogg = s->priv_data;
     struct ogg_stream *os = ogg->streams + idx;
 
-    uint64_t pts  = (granule >> 32);
+    int invcnt    = !((granule >> 30) & 3);
+    // If page granule is that of an invisible vp8 frame, its pts will be
+    // that of the end of the next visible frame. We substract 1 for those
+    // to prevent messing up pts calculations.
+    uint64_t pts  = (granule >> 32) - invcnt;
     uint32_t dist = (granule >>  3) & 0x07ffffff;
 
     if (!dist)