The engine now expects to emscripten FS to be setup and sync-ed before
main is called. This is exposed via `Module["initFS"]` which also allows
to setup multiple persistence paths (internal use only for now).
Additionally, FS syncing is done **once** for every loop if at least one
file in a persistent path was open for writing and closed, and if the FS
is not syncing already.
This should potentially fix issues reported by users where "autosave"
would not work on the web (never calling `syncfs` because of too many
writes).
