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.