Leiradel/savestate context rewind 2

[leiradel]

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Description

Added RETRO_SAVESTATE_CONTEXT_REWIND_SAME_INSTANCE for rewind savestate contexts.

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[barbudreadmon]

I tried investigating #16374 once again, and i still can't fully understand what's happening.
Adding more logs to FBNeo's retro_serialize/retro_unserialize shows that the size for each contexts are most likely correctly stored and used by RA (second arg size_t size is matching the expected size for the context).

My best guess is that the serialized data sent to retro_unserialize is wrong, because from what is happening visually in ga2, either FBNeo is receiving rewind's serialized data for the runahead context, or it receives a truncated version of runahead's serialized data (maybe because the ptr was allocated from the wrong size or something ?).

The problem is that RA's code is confusing, so i'm struggling figuring out what is used for what.