Fixes #2575.
Also removes unnecessary background texture (the actual storyboard background spec wasn't implemented correctly anyway).
Caused potential stutters for components that relied on this check.
GetStoryboard()
- [ ] Depends on https://github.com/ppy/osu-framework/pull/1035
Also ensures some extra fields inside Beatmap have non-null defaults.
Also fixes circular references when using IJsonSerializable.
Always parse storyboards for now. Let's not optimise this until it is necessary. It was leading to weird threading problems due to the load call in Player's async load method.