[effects] Delay taking screenshot into next rendering cycle

Summary:
For windows the screenshot effect already delayed the rendering into the
next cycle. But for the fullscreen variants which save to a file the
Screenshot effect did try to take the shot directly.

This can be problematic as e.g. the OpenGL context might not be bound at
all. Also this makes it impossible to screenshot multiple screens
together in the Wayland case.

This change delays the DBus replies, schedules the geometry and takes
the screenshot in the next rendering pass. It does not fix the
multiscreen wayland problem yet.

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D3052