UI: Emit DisplayResized() viewport signal

This allows knowing when the graphics viewport itself changes in size
(in case the window with the graphics viewport does not change size)

obs/qt-display.hpp

@@ -5,6 +5,15 @@
 class OBSQTDisplay : public QWidget {
 	Q_OBJECT
 
+	virtual void resizeEvent(QResizeEvent *event)
+	{
+		emit DisplayResized();
+		QWidget::resizeEvent(event);
+	}
+
+signals:
+	void DisplayResized();
+
 public:
 	inline OBSQTDisplay(QWidget *parent = 0, Qt::WindowFlags flags = 0)
 		: QWidget(parent, flags)