QtWayland: Graphical artifacts and hang ups when using nouveau

This is a continuation of

https://bugreports.qt.io/browse/QTBUG-73715

The symptoms are almost the same as described there, except that this bug is about artifacts and hang ups when using wayland. This time the bug is not a regression from 5.12.1 either, but has been present for quite some time.

The bug disappears when

007d73ef9e64a99e560cc932f519f8cb00121ba8

in qtwayland is reverted.

The working hypothesis is that the nouveau kernel driver cannot properly handle multithreaded OpenGL. Before 007d73ef9e64a99e560cc932f519f8cb00121ba8 that wasn't a problem since all of Mesa was blacklisted.

For X11 this fix was applied:

https://codereview.qt-project.org/c/qt/qtbase/+/254373

I believe a similar workaround needs to be found for wayland-egl, and (possibly xcomposite-egl?)?