Fix overdraw on Wayland
Since the DesktopView has "transparent" clear color, it will allocate a buffer with an alpha channel and QtWayland won't be able to set an opaque region. This change sets the clear color of DesktopView to "black" so overdraw can be fixed in kwin and to improve render performance on wayland. Cherry picked from commit 745e2c08, except that it also adds a platform check. Some people reported that setColor(Qt::black) in DesktopView breaks panel translucency on X11. A few of plasma developers have tried to reproduce that issue, but couldn't. It needs more investigation. In meanwhile, let's set clear color to black only on Wayland.