Leverage deduplication for prefixes to reduce wasted storage space

Feature Request

I confirm:

• that I have checked whether there are updates for my system available that contain this feature already.
• that I haven't found another request for this feature.

Suggestion

Instead of creating a prefix from scratch for each game, create a basic master prefix for each Proton version which you then only (sym-, hard-, ref-) link to for each game.

Justification [optional]

I've run rmlint -g on my currently ~30GB sized compatdata folder (68 prefixes) and it turned out that ~20GB of space are taken up by duplicate files. It would probably also speed up the first launch game setup.

Risks [optional]

Depending on which type of link you use to copy the files, certain risks present themselves. You might not want changes to the files in the master prefix to automatically propagate to all the linked game prefixes as it would with symlinks. So the most fitting link type would probably be reflinks, which only few file systems support (BTRFS). But you can just do cp --ref‐link=auto and it would already fall back on a plain copy, if the file system doesn't support reflinks.

No files, tests, or entry points are named. Start by locating the Proton code that creates and initializes per-game prefixes, then review the discussion around reflinks, fallback copies, and isolation between games. Done means prefixes can reuse a per-Proton-version master without unsafe propagation and retain a workable fallback on unsupported filesystems.