savegame: fix flame emitter item data save and load

[rr-]

Resolves #947

Checklist

• I have read the coding conventions
• I have added a changelog entry about what my pull request accomplishes, or it is an internal change

Description

Alternative implementation of #952. Rather than trying to rely on the order of loaded items, we reuse the calculated FX IDs and adjust item->data as necessary.

[walkawayy]

Crashes for me on reload.

1. new game
2. pull switch to turn off emitters
3. wait till switch comes up and emitters turn back on
4. save
5. load
6. pull switch and see if everything works