ui/previewer: remove the correct segment from the queue.

This fixes a bug that caused waveforms not to be drawn sometimes.

pitivi/ui/previewer.py

@@ -294,8 +294,7 @@ def _finishThumbnail(self, surface, segment):
         self._cache[segment] = surface
         self.emit("update", segment)
 
-        if segment in self._queue:
-            self._queue.remove(segment)
+        self._queue.remove(waiting)
         self._nextThumbnail()
         return False