Fix frame sync related to unprotected multithread access

There is a few crashes happens in real life that frame callback is double-free'd and hit an assertion in wayland-client. e.g. https://bugs.kde.org/show_bug.cgi?id=450003

This is due to the WaylandEventThread and calls to QWaylandWindow::reset may free and unset the mFrameCallback at the same time. mFrameSyncMutex should be used to protect such access.

Pick-to: 6.4 Change-Id: Ie01d08d07a2f10f70606ed1935caac09cb4f0382 (cherry picked from commit b6cbb5e323822d6e3ef5ed4dd5a4c4cc1ea85038)