ffmpeg: store divided frames in memory cache

Since users will most likely be working with a divider most of the time,
there's no reason to waste memory space with undivided frames considering
the dividing process is quite fast.

app/render/videoparams.h

@@ -114,6 +114,8 @@ public:
   static QStringList GetStandardPixelAspectRatioNames();
   static QString FormatPixelAspectRatioString(const QString& format, const rational& ratio);
 
+  static int GetScaledDimension(int dim, int divider);
+
 private:
   void calculate_effective_size();