fix(video): uses explicit default screen from QGuiApplication

This commit changes the method used to obtain the active screen from
simply selecting the first screen in the list to using the explicit
default screen from QGuiApplication::primaryScreen() function

src/video/cameradevice.cpp

@@ -133,7 +133,7 @@ CameraDevice* CameraDevice::open(QString devName, VideoMode mode)
         }
         else
         {
-            QScreen* defaultScreen = QApplication::screens().at(0);
+            QScreen* defaultScreen = static_cast<QGuiApplication*>(QGuiApplication::instance())->primaryScreen();
             qreal pixRatio;
 
 #if (QT_VERSION >= QT_VERSION_CHECK(5, 5, 0))