Attribute Qt::AA_ShareOpenGLContexts must be set before QCoreApplication is created.

QtWebEngine::initialize() expects to be called after a QApplication is created, and at the end it calls app->setAttribute(Qt::AA_ShareOpenGLContexts);: https://github.com/qt/qtwebengine/blob/5.12.4/src/core/api/qtwebenginecoreglobal.cpp#L130

However, in Qt 5.12.4, a warning was added when doing that, which is now triggered by QtWebEngine: https://codereview.qt-project.org/c/qt/qtbase/+/247206