Closes https://github.com/ppy/osu/issues/22802.
Stable uses a 1,000 ms fade-in / out. Rather than matching that, I've
stuck with 500ms (what lazer was already using for the fade-in) because
I think it feels better.
Tested using the beatmap linked in the issue thread.
Reasoning is explained in the inline comment (basically, stable doesn't
care what the user specifies as the frame count when falling back to
skin resources).
This change also removes on to two layers of drawables, which should be
a win in heavy storyboards.
Usually this would be handled by `TextureAnimation`, but because we are
inheriting from `DrawableAnimation` here for reasons, we needed to
implement this ourselves. Follows the implementation in
`TextureAnimation`.
In the case where the background image of individual difficulties is
different, querying the beatmap *set*'s metadata as we were will cause
issues.
I haven't added test coverage for this but can if required. Can be
manually tested using https://osu.ppy.sh/beatmapsets/1595773#osu/3377474
(specifically the highest difficulty).
Closes https://github.com/ppy/osu/discussions/16873.