Wayland clients flicker on NVIDIA hardware

When Qt 5.9.6 is used, flickering (frequent black frames) can be seen with the Wayland sample compositors qwindow-compositor and pure-qml and any Qt Wayland client (-platform wayland). Both, compositor and client have to use the threaded renderer. The same occurs on 5.12.0. Meanwhile there are following fixes available:

https://codereview.qt-project.org/244739
https://codereview.qt-project.org/244740
https://codereview.qt-project.org/244741

This is related to INC-1234510 and required for a customer product