Use a background thread to load ISO into RAM

[unknownbrackets]

This way we don't get slow startup. This will also cache the CSO data, for example, rather than the raw data, using up less RAM. It might even be reasonable to enable on 32-bit.

Ought to change the lang to remove "slow startup" if this is merged.

Makes #7951 not really an issue anymore, practically speaking.

Note that the semantics here are:

• It loads the ISO from start to finish.
• If the game tries to load farther ahead, it reads this synchronously, and then schedules a bit of readahead for the next few blocks.
• This still works with all file-loader types (e.g. even HTTP.)

-[Unknown]

[unknownbrackets]

Could probably refactor this logic into an HTTP downloader as well, since it would be the same concept.

-[Unknown]

[hrydgard]

I guess some people might still want to trade slow startup time for guaranteed no disk-hitches (these synchronous reads could potentially still take a little bit of time) but to me this makes a lot of sense.

[unknownbrackets]

Well, it does background read in chunks, and it only reads in 64k blocks. The hitches should be very minimal. Anyway, all they would have to do is wait the appropriate amount of time (the same time they would've waited before) and then there would be guaranteed no hitches.

-[Unknown]

[hrydgard]

Yeah, that's a good point, could just wait the same amount of time on the title screen indeed.

[Jack54guythecoder]

RIP to this feature, rest in spaghetti, never forgetti.

[hrydgard]

Actually, see #20158 . One assumption behind the removal was wrong, it wasn't broken/ineffective on all platforms, just on non-x86 (it broke on x86 fairly recently). So I'll probably revert.

[Unknowncat1991]

@hrydgard this Cache Full iso in RAM can also help chd file format? or only ISO or CSO only?
sorry for the stupid question 😅

[hrydgard]

Can in theory help either one, but on most machines you will not see any benefit. Only really useful for machines with slow harddrives that spin down.

Reverting in #20165 , this time with correct platform checks.