Closes https://github.com/ppy/osu/issues/25263.
In some circumstances, stable allows skipping twice if a particularly
long storyboarded intro is being displayed:
https://github.com/peppy/osu-stable-reference/blob/master/osu!/GameModes/Play/Player.cs#L1728-L1736
`AllowDoubleSkip` is calculated thus:
3ea48705eb/osu!/GameModes/Play/Player.cs#L1761-L1770
and `leadInTime` is calculated thus:
3ea48705eb/osu!/GameModes/Play/Player.cs#L1342-L1351
The key to watch out for here is `{first,last}EventTime`. `EventManager`
will calculate it on-the-fly as it adds storyboard elements:
3ea48705eb/osu!/GameplayElements/Events/EventManager.cs#L253-L256
However, this pathway is only used for sprite, animation, sample,
and break events. Video and background events use the following pathway:
https://github.com/peppy/osu-stable-reference/blob/master/osu!/GameplayElements/Events/EventManager.cs#L368
Note that this particular overload does not mutate either bound.
Which means that for the purposes of determining where a storyboard
starts and ends temporally, a video event's start time is essentially
ignored.
To reflect that, add a clause that excludes video events from
calculations of `{Earliest,Latest}EventTime`.
9ce2c1f49c