Summary:
It is not clear what the advantage of triple buffering is for KWin. An X11
compositor is meant to swap buffers once every monitor cycle. For that triple
buffering is not necessary.

The functionality is not maintained, does not reliably work as displayed by
the existence of an environment variable to force some behavior, pollutes
our code and every compositing-related problem that might be mitigated with
triple buffering should find a simpler and more fitting solution with other
means.

There is one caveat which is if we shall block for retrace. We set it
currently according to the result of the swap profiler and in the most common
case with double buffering it is set to true. But on Nvidia systems this might
be actual the wrong behavior. Instead of trying to work around this ignore
the issue for now and move the overall architecture to something less complex
by presenting after paint how we do it in the Wayland DRM backend and with
double buffering on GLX (although this is at the moment also borken because
we actually present then twice).

Test Plan: kwin_x11 tested on i915.

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: zzag, fredrik, kwin

Tags: #kwin

Maniphest Tasks: T11071

Differential Revision: https://phabricator.kde.org/D23504