subtitles: implemented very basic renderer

app/node/project/sequence/sequence.cpp

@@ -118,6 +118,15 @@ void Sequence::Retranslate()
   }
 }
 
+void Sequence::InvalidateCache(const TimeRange &range, const QString &from, int element, InvalidateCacheOptions options)
+{
+  if (from == kTrackInputFormat.arg(Track::kSubtitle)) {
+    emit SubtitlesChanged(range);
+  }
+
+  super::InvalidateCache(range, from, element, options);
+}
+
 rational Sequence::VerifyLengthInternal(Track::Type type) const
 {
   if (!track_lists_.isEmpty()) {

app/node/project/sequence/sequence.h

@@ -84,6 +84,8 @@ public:
 
   virtual void Retranslate() override;
 
+  virtual void InvalidateCache(const TimeRange& range, const QString& from, int element, InvalidateCacheOptions options) override;
+
   static const QString kTrackInputFormat;
 
   virtual bool IsItem() const override
@@ -102,6 +104,8 @@ signals:
   void TrackAdded(Track* track);
   void TrackRemoved(Track* track);
 
+  void SubtitlesChanged(const TimeRange &range);
+
 private:
   QVector<TrackList*> track_lists_;