Preemptive Frames: Relax savestate requirement to 'serialized'

[neil4]

Description

Draft PR for testing Preemptive Frames with its savestate requirement relaxed from 'deterministic' to 'serialized'. This makes it available to cores which likely have never been tested before with RunAhead (or Preemptive Frames).

Cores especially worth testing for speed are Beetle PSX HW, Parallel N64, Mupen, and Beetle Saturn.

Reviewers

@LibretroAdmin

[hizzlekizzle]

Many cores "work" with runahead with serialized states, too, but you can get weird rollback divergence effects due to the lack of determinism, so we disable it to be extra-safe. I don't know if this is also the case with preemptive frames, but it's something to look out for.

[Tatsuya79]

I suggest this instead: #14906

A simple setting to allow every experimentation, disregarding the info files.

[neil4]

Reopening - I mistook #14906 to be a replacement for this PR. Depending on how testing goes, it might make sense to change preemptive frames' default requirement.

[LibretroAdmin]

Hi @neil4 , sorry for taking almost a year for this PR. If we can resolve the merge conflict I'd be happy to merge this now.

Again, my bad, feel very bad for letting this PR languish for so long but would be totally willing to fasttrack it now.

[neil4]

No worries, I'll look at this again pretty soon. I've also made a few updates recently to preemptive frames that I need to bring over.