Fix MissingResource properties being stripped on save

[nikitalita]

fixes #86599

See #60597 (review)

There were a number of bugs in the MissingResource implementation that prevented it from actually preserving the missing resources when saving; I have corrected these.

[fire]

@reduz Preserving the missing resources when saving is a good thing, but I would like a technical second look.

[fire]

Can you do a rebase? I'll try to review.

[nikitalita]

Thanks, I've rebased on current master

[nikitalita]

@fire I've rebased again, can you take a look?

[fire]

I think it's ok, but it's also touching core so I'm a bit wary.

Will try it on my projects.

[AThousandShips]

Style looks good

[hpvb]

LGTM

[lyuma]

The bug and this PR are fixing an issue "stripped upon save". Why then is it modifying load-side functions ResourceLoader and scenestate::_parse_node?

i am concerned that this implies some form of compat breakage (resources changing next time I update the engine)

[nikitalita]

The bug and this PR are fixing an issue "stripped upon save". Why then is it modifying load-side functions ResourceLoader and scenestate::_parse_node?

i am concerned that this implies some form of compat breakage (resources changing next time I update the engine)

Please see here for a full explanation, but basically:

The reason for ResourceLoader being modified is that the pre-existing missing resource assignment was buggy; thus, the missing resources were not actually being loaded as MissingResources, and as such were not being saved when the scene/resource was re-saved.

The reason for scenestate::_parse_node being modified:

Nodes report their theme override properties as having PROPERTY_USAGE_STORAGE based on whether or not they're actually set. If a theme override property was set to a missing resource, it will be reported as not having PROPERTY_USAGE_STORAGE and it will be skipped. This should check if missing_resource_properties has the property.

Basically, when saving a scene, nodes that had properties set to a missing resource were skipped because they weren't previously loaded into the packed scene.

This shouldn't mess with scene instancing. During actual instancing, when attempting to assign a MissingResource to a node property, it'll just silently fail.

[Repiteo]

Thanks!