Reduce lag when changing patch in the editor caused by background image loading

[hhyyrylainen]

by background loading (if / when that is possible in godot) the images and only then applying the background for the camera.
The background setting method could be changed to take in Textures.

[hhyyrylainen]

Background image loading seems a bit problematic: #2078

Perhaps Godot 4.0 will improve the situation?

[hhyyrylainen]

I added a bunch of scheduling code for resource loading, which should make it pretty easy to make the lag spike lessened: #3682 (still can't fully move things to background thread before Godot 4.0).