QQuickWidget and QOpenGLWidget broken with toplevels that have alpha enabled

QOpenGLWidget and QQuickWidget give weird results on systems where the system gives a context/surface with alpha.

For example with Mesa/Intel configured for EGL/GLES2, the window always gets a 32-bit visual. This results in something like the attached screenshot. Neither the transparency nor the swapped red and blue are ok.

The same issue is visible when explicitly requesting alpha for the toplevel via setAttribute(Qt::WA_TranslucentBackground).

This is probably a side effect of how qplatformbackingstore performs the composition.