Increase loading performance about openatdeluxe

[Serphentas]

(Quick parenthesis: I was unavailable these days so not many contributions, sorry)

Good practise is to load content when needed, but given the game's rather small size it could possibly be done upon startup. Do you have some RAM figures ?

If this approach is not feasible, I could look into multi-threaded ops.

[WizzardMaker]

(No problem, there is no rush! We all have a life)

The Airport and some of the rooms use according to the Godot debugger only 38MB, but thats with only 3 out of ca. 24 rooms, though the biggest RAM eater is the airport. I'm planning on adding all the rooms later this day to get the final amount.

[WizzardMaker]

Just for reference, the current game has a RAM footprint of around 100.83 MB