Rearchitect cutscenes

[nic-hartley]

Cutscenes are currently kinda grody blobs of code that just get an IoSystem and go to town. That makes the experience hard to keep standard, because anything I implement in the base game has to be manually implemented in cutscenes too. It also makes them less compatible with the Game/Gameplay system I'm trying to build, which periodically asks the game to render itself rather than the game entirely controlling the IoSystem.

There are two decent options, as I see it:

• Add a special carveout for cutscenes and bake them into the engine as a first-class feature, which gets exclusive control over the IoSystem while it's running. Cutscenes still largely have to reimplement things, but they're at least compatible with the Game/Gameplay system.
• Write a different async-ish executor for IO stuff (maybe with Screen::render being the only awaitable? or a new one, "frame"/"io"/whatever?) to give the engine a hook for all its orchestration.