render: improved overall flow

app/node/traverser.cpp

@@ -260,6 +260,7 @@ NodeValueTable NodeTraverser::ProcessInput(const Node* node, const QString& inpu
 
 NodeTraverser::NodeTraverser() :
   cancel_(nullptr),
+  heard_cancel_(false),
   transform_(nullptr)
 {
 }

app/node/traverser.h

@@ -128,11 +128,17 @@ protected:
 
   QVector2D GenerateResolution() const;
 
-  bool IsCancelled() const
+  bool IsCancelled()
   {
-    return cancel_ && *cancel_;
+    bool c = cancel_ && *cancel_;
+    if (c) {
+      heard_cancel_ = true;
+    }
+    return c;
   }
 
+  bool HeardCancel() const { return heard_cancel_; }
+
   const QAtomicInt *GetCancelPointer() const
   {
     return cancel_;
@@ -155,6 +161,7 @@ private:
   AudioParams audio_params_;
 
   const QAtomicInt *cancel_;
+  bool heard_cancel_;
 
   const Node *transform_start_;
   const Node *transform_now_;