Automatically enable headless mode when exporting

[KoBeWi]

Fixes #43752

[beicause]

Does this cause #94734? Also shader baker don't seem to work in headless mode (#102552 (comment))

[KoBeWi]

Does this cause #94734?

It's the same as running export with --headless, so likely yes.
Headless mode has improved a lot since the last time this issue was being fixed, but seems like it still has problems and not sure if they will ever get fully resolved.

I guess the options are:

• Make headless opt-out when exporting
• Only do auto-headless when not using --import
• Close the issue as WONTFIX

[Calinou]

I would prefer we implement godotengine/godot-proposals#5790, then make --import and --export always use off-screen rendering. This way, operations that require a GPU keep working, but you don't see any visible editor window during export.

When doing so, we should make sure that using the offscreen DisplayServer should use the same CLI progress as headless.