MainWindow: fix window redraw after new project

Canvas is not initialized in application start so a window manager like x11 may reopen the mainwindow on gl context switching without this fix.

The behaviour was caused by different surface format / opengl contexts of the QMainWindow instance and the QOpenGLWidget instance of Canvas. In Qt6 it became more complex to quickly share the gl context. Hence we just create a dummy QOpenGLWidget before MainWindow is show to reset it.

GUI/View/Main/MainWindow.cpp

@@ -31,6 +31,7 @@
 #include <QButtonGroup>
 #include <QCloseEvent>
 #include <QMessageBox>
+#include <QOpenGLWidget>
 #include <QProgressBar>
 #include <QPushButton>
 #include <QSettings>
@@ -74,6 +75,8 @@ MainWindow::MainWindow()
     vlayout->setSpacing(0);
     vlayout->addLayout(m_viewsStack);
 
+    vlayout->addWidget(new QOpenGLWidget()); // quick fix to reset surface format
+
     mainLayout->addLayout(m_viewSelectionButtonsLayout);
     mainLayout->addLayout(vlayout);