time is expressed from the start of the current segment, so always

 record the start to offset from, not just the first time, or seeking
 will cause reported position to be wrong.

src/com/fluendo/plugin/OggDemux.java

@@ -121,8 +121,7 @@ public class OggDemux extends Element
       if (!active)
         return;
 
-      if (baseTs == -1)
-	baseTs = firstTs;
+      baseTs = firstTs;
 
       time = firstTs - baseTs;