Add a helper function that ensures any queued virtual input events have
been flushed from the input thread. This works by posting a task to the
input thread, which will itself queue another callback back to the main
thread. Once the main thread callback is invoked, the flush call is
unblocked and the function returns. Upon this, any previously emitted
virtual input event should have already passed through the input thread
back into the main thread, however not necessarily fully processed.
For making sure it has been processed, one also have to make sure the
stage has been updated, e.g. via `meta_wait_for_paint()`.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2727>
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