qt: fix crash when trying to load video before interface is ready

This method requires the direct composition device in order to create the video visual.

When this method is called, Qt might not have fully initialized the interface as scene graph initialization is done asynchronously.

In this case, simply wait for the initialization to complete. If the initialization fails or takes too much time to complete, then fail by returning false.

Regression since !5950 (merged).

Request review @chub.