QtWidgets Vulkan RHI backend doesn't support translucency on Wayland

I applied the following patches on top of 6.4.0 beta2:

1. https://codereview.qt-project.org/c/qt/qtbase/+/421794
2. https://codereview.qt-project.org/c/qt/qtbase/+/422057

Now that I can run the default RHI configuration without any issues, I decided to go further and try Vulkan backend. Unfortunately, windows with WA_TranslucentBackground don't have translucency, just like on X11 without the first patch. QOpenGLWidgets lose the translucency in this configuration, too.