ensured renders can be cancelled

dialogs/exportdialog.cpp

@@ -529,6 +529,7 @@ void ExportDialog::update_progress_bar(int value, qint64 remaining_ms) {
 }
 
 void ExportDialog::on_renderCancel_clicked() {
+    panel_sequence_viewer->viewer_widget->force_quit = true;
 	et->continueEncode = false;
     cancelled = true;
 }

panels/viewer.h

@@ -57,7 +57,7 @@ public:
 	bool is_recording_cued();	
 	long recording_start;
 	long recording_end;
-	int recording_track;
+    int recording_track;
 
 	void reset_all_audio();
     void assert_audio_device();

ui/viewerwidget.cpp

@@ -736,6 +736,7 @@ GLuint ViewerWidget::compose_sequence(QVector<Clip*>& nests, bool render_audio)
 
 void ViewerWidget::paintGL() {
     drawn_gizmos = false;
+    force_quit = false;
     if (viewer->seq != NULL) {
         gizmos = NULL;
 
@@ -784,6 +785,7 @@ void ViewerWidget::paintGL() {
                 p.drawLine(playhead_x, 0, playhead_x, height());
             }
 
+            if (force_quit) break;
             if (texture_failed) {
                 if (rendering) {
                     dout << "[INFO] Texture failed - looping";

ui/viewerwidget.h

@@ -33,6 +33,8 @@ public:
 	bool waveform;
 	Clip* waveform_clip;
 	MediaStream* waveform_ms;
+
+    bool force_quit;
 public slots:
     void delete_function();
 protected: