qt: fix use after free in `CompositorVideo`

This is intended to replace !7195 (closed) without the controversial changes.

I initially wanted to call destroyCb() in a queued call, so that CompositorVideo::windowDestroy() returns before the CompositorVideo instance is potentially destroyed. However, the IsoCpp FAQ finds it not evil, so I did not do that:

"Is it legal (and moral) for a member function to say delete this?" "As long as you’re careful, it’s okay (not evil) for an object to commit suicide (delete this)."

Request review @chub.