DrawableStoryboard
allowFallback
This allows the `FillMode.Fill` to take up the full space of the storyboard container.
This matches stable behaviour, which will allow videos to display filling the screen if they are the only thing contained within the "storyboard".
We can call the base method regardless for better safety. Worst case it's just going to run `Stop()` twice anyway.
Allows sharing pause logic with gameplay samples.