A couple of enhancements to user-visible threading semantics

[RandomShaper]

Kept in two commits because, regardless both related to threading, the changes are heavily unrelated otherwise.

I'd appreciate especially careful reviewing of my GDVIRTUAL changes, as this is the first time I deal with that.

[RandomShaper]

CC @SlugFiller

[SlugFiller]

Cool! Nice to see things are happening quickly. Here's hoping it gets merged soon.

[YuriSizov]

Here's hoping it gets merged soon.

Did you test it, do you approve it? 🙃

[SlugFiller]

With just the first commit, I hit the same deadlock in running ResourceSaver.save in parallel.

With both commits, and _can_import_threaded set to return false, it works perfectly. However, if I don't implement _can_import_threaded, Godot poofs on import. So something is definitely not right about the GDVIRTUAL_CALL.

[RandomShaper]

@SlugFiller, thank you for the feedback. I'm wondering, what happens if you override but return true?

[SlugFiller]

When returning true it works with single, or deadlocks on ResourceSaver.save if done in parallel. If I don't override it, it poofs even on single.

[RandomShaper]

I've fixed a very silly mistake causing infinite recursion if the method is not overridden. Please test again with false. The problem with parallel saving will have to be investigated and fixed separately as it's not in the scope of this PR.

[SlugFiller]

Poofing is gone, and returning false works for importing 3 scenes at once. Seems great.

[akien-mga]

Thanks!