[plugins/qpa] Implement native offscreen surface

Summary:
Depending on whether the underlying platform supports offscreen surfaces,
QOffscreenSurface may create an invisible QWindow. In our case that's the
case, for each offscreen surface a native window is created. This may
lead to some funky results related to window decorations, see bug 407612.

There are several ways to implement offscreen surfaces - either use pbuffers
or utilize a surfaceless context extension. For the sake of simplicity
this change sticks with pbuffers, but it's a good idea to support both
methods.

CCBUG: 407612

Reviewers: #kwin, romangg

Reviewed By: #kwin, romangg

Subscribers: romangg, alexeymin, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D22150