audiocache: ensure cached ranges are actually marked as validated

For some reason, the audio cache never validated itself leading to an
endless loop of trying to re-render it. This is fixed now.

app/render/audioplaybackcache.cpp

@@ -81,6 +81,8 @@ void AudioPlaybackCache::WritePCM(const TimeRange &range, SampleBufferPtr sample
     }
 
     f.close();
+
+    Validate(range);
   } else {
     qWarning() << "Failed to write PCM data to" << filename_;
   }