Make DesktopView set black clear color only on Wayland
745e2c08 made DesktopView set clear color to "black" so the compositor could avoid overdraw and thus optimizing rendering. In the perfect world, such a change would create no regressions, but for some reason, it made panels lose alpha channel on X11.
That makes absolutely no any sense. DesktopView and PanelView are two totally different code paths. So changes in DesktopView should not affect the PanelView. This looks like a QtXCB bug.
Ideally, we should add workarounds only after finding what creates a problem, but this one is particularly hard because many people who have tried to reproduce that issue weren't successful. Somebody who can reproduce the issue would have to be able to build Qt from source code and add qDebug()s. These requirements are too steep, thus adding a platform check seems like the best option at the moment.