Fix some VFS issues

[digant73]

Fix some issues on VFS and optimize iso access:

• Fix: raw-device games folder is written to games.yml without its root delimiter. With the VFS games folder set to a BD drive root (I:/), the entry was stored as I: and then alternated with I:/ on every game-list refresh
• rpcs3/Emu/System.cpp — a game_found() predicate (no_errors || already_added) now guards both the subfolder descent and the recursive scan, so a path that is already a registered game is not re-registered through a different route.
• For a raw device that is every refresh (both caches are bypassed by design). add_game() now takes an optional shared_ptr<iso_archive> and skips its own is_iso_file() when given one.
• New ISO_DESCRIPTORS_OFFSET (which also replaces the two 32768 literals) and a seek to it
• A std::regex built per directory entry (system_utils.hpp:84). ^PS3_GM[[:digit:]]{2}$ appeared in four places, three of them constructing the regex inside the loop body (AddGame, GetBdvdDir, add_disc_dir). All four now call one inline
• AddGamesFromDir saved games.yml inside its own recursion (System.cpp:4416, :4493, games_config.h:13). The function is recursive but restored set_save_on_dirty(true) unconditionally, so with use_recursive_scan each nested call wrote the whole file on return and left the per-entry write enabled for the rest of the outer scan — a full YAML emit + pending_file rename per added game. Now the previous value is saved/restored (new is_save_on_dirty()) and the flush happens only at the outermost level.

[Megamouse]

So if this is false on the first call then nothing is saved ?

[digant73]

well, yeah. but that should mean someone else should save it. I do not see anything leaving the state to false