lib/xiph/OggVisitor: update the OggStreamState offset

src/decoder/plugins/OggDecoder.cxx

@@ -72,7 +72,7 @@ inline void
 OggDecoder::SeekByte(offset_type offset)
 {
 	input_stream.LockSeek(offset);
-	PostSeek();
+	PostSeek(offset);
 }
 
 void

src/lib/xiph/OggVisitor.cxx

@@ -94,9 +94,10 @@ OggVisitor::Visit()
 }
 
 void
-OggVisitor::PostSeek()
+OggVisitor::PostSeek(uint64_t offset)
 {
 	sync.Reset();
+	sync.SetOffset(offset);
 
 	/* reset the stream to clear any previous partial packet
 	   data */

src/lib/xiph/OggVisitor.hxx

@@ -53,12 +53,18 @@ public:
 		return stream.GetSerialNo();
 	}
 
+	uint64_t GetStartOffset() const noexcept {
+		return sync.GetStartOffset();
+	}
+
 	void Visit();
 
 	/**
 	 * Call this method after seeking the #Reader.
+	 *
+	 * @param offset the current #Reader offset
 	 */
-	void PostSeek();
+	void PostSeek(uint64_t offset);
 
 private:
 	void EndStream();