fn rav1d_worker_task: Refactor state machine

[rinon]

rav1d_worker_task involves complex control flow with gotos, and therefore was transpiled into a state machine. This refactoring restores the original structure with minimal additional structured control flow to emulate gotos.

[kkysen]

I think https://github.com/memorysafety/rav1d/pull/671/files#r1449731224 is the only thing that's wrong, though I made some other suggestions that would IMO greatly help readability.

[rinon]

Thanks for the useful feedback, I've applied everything except for the labeled block for found, just want to avoid extra indentation if we can. I forgot that labelled blocks were added, but that cleans up the goto next code perfectly, thanks.

[kkysen]

There's that one // next: comment I think you should add back, but otherwise everything LGTM now. I slightly prefer actual labeled blocks vs. a // label: comment, but the indentation isn't great either, so it's fine as is.