Gui: Force OpenGL rendering for main window

This is big hack for Qt6 to force rendering using OpenGL from the
beginning. Since Qt6 OpenGL context is initialized only after creating
first QOpenGLWidget which results in window hiding and showing again
after some delay. This causes various timing issus as the window is not
supposed to do that.

src/Gui/MainWindow.cpp

@@ -392,6 +392,14 @@ MainWindow::MainWindow(QWidget * parent, Qt::WindowFlags f)
     d->whatsthis = false;
     d->assistant = new Assistant();
 
+#if (QT_VERSION >= QT_VERSION_CHECK(6,0,0))
+    // this forces QT to switch to OpenGL mode, this prevents delay and flickering of the window
+    // after opening project and prevent issues with double initialization of the window
+    //
+    // https://stackoverflow.com/questions/76026196/how-to-force-qt-to-use-the-opengl-window-type
+    new QOpenGLWidget(this);
+#endif
+
     // global access
     instance = this;