rational: implement NaN mode

Also clarify code where 0 is used as a rational.

app/render/framehashcache.cpp

@@ -322,7 +322,7 @@ void FrameHashCache::ShiftEvent(const rational &from, const rational &to)
   // POSITIVE if moving forward ->
   // NEGATIVE if moving backward <-
   rational diff = to - from;
-  bool diff_is_negative = (diff < rational());
+  bool diff_is_negative = (diff < 0);
 
   QList<HashTimePair> shifted_times;
 
@@ -353,10 +353,12 @@ void FrameHashCache::ShiftEvent(const rational &from, const rational &to)
 
 void FrameHashCache::InvalidateEvent(const TimeRange &range)
 {
-  QVector<rational> invalid_frames = GetFrameListFromTimeRange({range});
+  if (!timebase_.isNull()) {
+    QVector<rational> invalid_frames = GetFrameListFromTimeRange({range});
 
-  foreach (const rational& r, invalid_frames) {
-    time_hash_map_.remove(r);
+    foreach (const rational& r, invalid_frames) {
+      time_hash_map_.remove(r);
+    }
   }
 }
 

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)
   {
   }
 

app/render/previewautocacher.cpp

@@ -18,8 +18,7 @@ PreviewAutoCacher::PreviewAutoCacher() :
   last_update_time_(0),
   ignore_next_mouse_button_(false)
 {
-  // Set default autocache range
-  SetPlayhead(rational());
+  SetPlayhead(0);
 
   delayed_requeue_timer_.setInterval(Config::Current()[QStringLiteral("AutoCacheDelay")].toInt());
   delayed_requeue_timer_.setSingleShot(true);