rational: implement NaN mode

Also clarify code where 0 is used as a rational.

app/render/playbackcache.h

@@ -35,7 +35,8 @@ class PlaybackCache : public QObject
   Q_OBJECT
 public:
   PlaybackCache(QObject* parent = nullptr) :
-    QObject(parent)
+    QObject(parent),
+    length_(0)
   {
   }