lib/xiph/OggVisitor: add method ReadGranulepos()

src/lib/xiph/OggVisitor.cxx

@@ -108,3 +108,14 @@ OggVisitor::PostSeek(uint64_t offset)
 
 	post_seek = true;
 }
+
+ogg_int64_t
+OggVisitor::ReadGranulepos() noexcept
+{
+	ogg_packet packet;
+	while (stream.PacketOut(packet) == 1)
+		if (packet.granulepos >= 0)
+			return packet.granulepos;
+
+	return -1;
+}

src/lib/xiph/OggVisitor.hxx

@@ -66,6 +66,22 @@ public:
 	 */
 	void PostSeek(uint64_t offset);
 
+	/**
+	 * Skip packets (#ogg_packet) from the #OggStreamState until a
+	 * packet with a valid granulepos is found or until the stream
+	 * has run dry.
+	 *
+	 * Since this will discard pending packets and will disturb
+	 * this object, this should only be used while seeking.
+	 *
+	 * This method must not be called from within one of the
+	 * virtual methods.
+	 *
+	 * @return the granulepos or -1 if no valid granulepos was
+	 * found
+	 */
+	ogg_int64_t ReadGranulepos() noexcept;
+
 private:
 	void EndStream();
 	bool ReadNextPage();