Remove dependency on deprecated QtOpenGL module

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This class is obsolete.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`.

First locate the use of QtOpenGL and QGLWidget::convertToGLFormat in the C++ codebase; the issue names no file or test. Confirm that the replacement preserves the stated vertical mirror and BGRA-to-RGBA channel flipping, then verify the existing rendering or image-handling tests if available.

Written by the indexing model from the issue text.