Draft: qt: synchronize video window when interface frame is queued

This should reduce the period where synchronization between the interface and the video window is broken.

I mark this as a draft for now, comments would be appreciated @chub @alexandre-janniaux. I wanted to do this for a while, but now I had the time to propose it.

For context (https://doc.qt.io/qt-6/qtquick-visualcanvas-scenegraph.html):