Summary:
The syncing of the window geometry to the internal geometry can
unfortunately cause a freeze in very special conditions:
1. create QML component
2. a Plasma::Dialog gets created
3. It creates the DialogShadows
4. This triggers QGlobalStatic creation which locks a non-recursive
mutex
5. The creation of DialogShadows creates a Registry and triggers a
roundtrip on the Wayland server
6. KWin processes all Wayland events
7. This triggers the creation of a ShellClient
8. The ShellClient has a PlasmaShellSurface which requested a position
9. The new geometry does not match the geometry of the Plasma::Dialog
10. ShellClient syncs the geometry to the Plasma::Dialog
11. Plasma::Dialog updates the theme because window geometry changed
12. This accesses the DialogShadows...

which is still in the non recursive mutex and we have a freeze.

By delaying the sync to the end of cycle we jump out of this deadly
sequence.

BUG: 384441

Test Plan:
The freeze doesn't hit any more. It's possible that some test
cases need adjustments.

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D7712