Handle explicitly unthrottled windows better in the threaded render loop

When vsync is requested to be disabled for a QQuickWindow explicitly, the logic in the threaded render loop can easily recognize this and fall back to the system timer based way of driving animations. (i.e. behave exactly as if there were more than 1 windows visible, which is another case where driving animations by vsync would not work)

Note that this would just handle the case where the window has disabled vsync explicitly in the application or via the environment variable (QTBUG-99950).

This can be extended separately with something smarter that tries to recognize the lack of throttling when the application did not request, but the system does do unthrottled presentation. That's not in scope here though and lives in QTBUG-99952 instead.