Handle restart of Compositor Scene correctly for Wayland client

Summary:
This change ensures that KWin doesn't crash in the QPainter scene if
the compositor gets restarted and if there are Wayland clients.

Cherry-picked from master

BUG: 365471
FIXED-IN: 5.7.3

Test Plan:
Test case added to scene qpainter which triggers a restart
of the Compositor with a window being shown. Verifies that rendering
is correct afterwards.

Reviewers: KWin, Plasma on Wayland

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D2185