viewer: default to black background

Fixes theme background no null frame in full screen.

app/widget/viewer/viewer.cpp

@@ -72,6 +72,7 @@ ViewerWidget::ViewerWidget(QWidget *parent) :
 
   display_widget_ = new ViewerDisplayWidget();
   display_widget_->setAcceptDrops(true);
+  display_widget_->SetShowWidgetBackground(true);
   connect(display_widget_, &ViewerDisplayWidget::customContextMenuRequested, this, &ViewerWidget::ShowContextMenu);
   connect(display_widget_, &ViewerDisplayWidget::CursorColor, this, &ViewerWidget::CursorColor);
   connect(display_widget_, &ViewerDisplayWidget::ColorProcessorChanged, this, &ViewerWidget::ColorProcessorChanged);

app/widget/viewer/viewerdisplay.cpp

@@ -49,7 +49,8 @@ ViewerDisplayWidget::ViewerDisplayWidget(QWidget *parent) :
   hand_dragging_(false),
   deinterlace_(false),
   show_fps_(false),
-  frames_skipped_(0)
+  frames_skipped_(0),
+  show_widget_background_(false)
 {
   connect(Core::instance(), &Core::ToolChanged, this, &ViewerDisplayWidget::UpdateCursor);
 
@@ -301,7 +302,7 @@ void ViewerDisplayWidget::dropEvent(QDropEvent *event)
 void ViewerDisplayWidget::OnPaint()
 {
   // Clear background to empty
-  QColor bg_color = palette().window().color();
+  QColor bg_color = show_widget_background_ ? palette().window().color() : Qt::black;
   renderer()->ClearDestination(bg_color.redF(), bg_color.greenF(), bg_color.blueF());
 
   // We only draw if we have a pipeline

app/widget/viewer/viewerdisplay.h

@@ -71,6 +71,12 @@ public:
   void SetVideoParams(const VideoParams &params);
   void SetTime(const rational& time);
 
+  void SetShowWidgetBackground(bool e)
+  {
+    show_widget_background_ = e;
+    update();
+  }
+
   FramePtr last_loaded_buffer() const;
 
   /**
@@ -288,6 +294,8 @@ private:
   QVector<double> frame_rate_averages_;
   int frame_rate_average_count_;
 
+  bool show_widget_background_;
+
 private slots:
   void EmitColorAtCursor(QMouseEvent* e);