Improve thread safety of resource loading

[RandomShaper]

During resource loading, including threaded loading, certain resources want to emit a changed signal. Since most of the Object API, including signals, is not thread-safe, there's the risk of this causing issues. I've actually seen it happening, but I can't be sure if not getting them anymore is due to this fix.

This PR will defer any emissions of that kind involved in loading that happen on a non-main thread. UPDATE: Bear in mind that during threaded loading, deferred calls are even more deferred, queued in a thread-specific call queue, until the resource is done loading, then flushed on the main thread.

UPDATE 2: An alternative would be letting Object itself defer signal-related calls during loads. However, I'm a bit afraid of adding a check there is too heavy for what so far seem to be few cases (more to discover, likely). Additionally, maybe at some point Object has to be made entirely thread-safe due to popular demand or something, and patches like this are not needed anymore.

[KoBeWi]

Shader changes cause uniform values to not load properly at runtime:

2INkzzPfGe.mp4

BrokenShader.tscn.txt

[RandomShaper]

@KoBeWi, thanks for reviewing. Can you confirm you have tested with the latest push I made to this branch? I'm asking because I realized issues and pushed fixes.

If you are already testing the latest, please share the MRP. I need it because with the latest changes I'm not seeing issues anymore on my side.

[KoBeWi]

It was with latest changes and I already shared MRP.

[RandomShaper]

It was with latest changes and I already shared MRP.

I've been like reliable research, that is, doubly-bind:

1. I couldn't spot the MRP.
2. When I re-pushed to this branch, I did it from a source branch that hadn't been updated with the fixes.

It should be OK now.

[KoBeWi]

I did some testing and it doesn't seem to break anything now, but I'm not sure how to test whether it fixes something.

[RandomShaper]

The way I realized the issue was using multi-threaded loading, with ShaderMaterials involved. On many runs I was getting different issues, the most frequent being a failed dev assertion in Object::emit_signal() or different kinds of crashes.

[akien-mga]

Thanks!