Remove dependency on deprecated QtOpenGL module

[j-o]

From http://doc.qt.io/qt-5/qglwidget.html:

This class is obsolete. It is provided to keep old source code working.
We strongly advise against using it in new code.

It is only need for QGLWidget::convertToGLFormat, which boils down to a vertical mirror and channel flipping from BGRA to RGBA.

[sbusch42]

Sure, if someone likes to provide an implementation for that, so that we can remove this dependency, please go ahead :)