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

composite.cpp

@@ -322,6 +322,13 @@ void Compositor::startupWithWorkspace()
         c->setupCompositing();
         c->getShadow();
     }
+    if (auto w = waylandServer()) {
+        const auto clients = w->clients();
+        for (auto c : clients) {
+            c->setupCompositing();
+            c->getShadow();
+        }
+    }
 
     emit compositingToggled(true);