Summary:
Plasmashell's desktop windows are RGBA which forces the compositor to
perform blending and render the background. That is absolutely pointless
as there is no window behind the desktop window it could blend to. All it
does is destroying KWin's more optimized code path and forcing additional
rendering which will never be visible (including shader push/pop).

With this change KWin forces desktop windows (both X11 and Wayland) to
be considered as opaque by setting the depth to 24. Thus blending is
disabled and the background is not rendered.

Test Plan:
Verified with apitrace that KWin goes in the opaque rendering
path for desktop windows.

Reviewers: #kwin, #plasma

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D2382