Store hard drive diffs in some way

In cases when the game has to be installed onto a hard drive image, it may be legally iffy to allow freely sharing the movie file containing the diff as a SaveRAM anchor.

And it may be too huge to even upload to tasvideos (up to hundreds of MBs if we only take Amiga, but also some MAME games need installation, and if we ever emulate IBM PC then we're talking gigs).

My initial thought was to keep SaveRAM for those movies that depend on prior installation separately from the movie, and pull from disk as needed. There's some issue with it that I'd like CPP to explain.

But if diff is actually written to the file it applies to (as libTAS does it), that also sounds like a great solution, even tho ideally it'd be kept separately and applied on the fly in memory.

Also ideally there'd be a libTAS like option to not touch the file and to only keep the diff in memory, and to kill it once the game is closed, instead of flushing.

Start by tracing BizHawk's existing SaveRAM and movie diff handling, especially how installed hard-drive images are represented. Compare the possible separate-storage, in-place, and memory-only approaches described in the issue, then define the storage and cleanup behavior before implementation.