[3.x] Fix editor potentially having multiple instances of the same subresource

[RandomShaper]

This fixes a nasty bug I've been having.

The issue can be reproduced as follows:

1. Create the scene inheritance chain A -> B -> C.
2. Put some kind of node in A with a subresource in a property. Save.
3. Open C.
4. Close A. Re-open A.
5. Save C.

You'll see that C now contains a copy of the subresource in A.

The editor loads scenes ignoring the cache, but subresources must still use it, because otherwise loading a higher-level scene will cause new instances of its subresources to be created, while deeply derived scenes will keep pointing to the ones before the load of the top scene.

This PR adds separate cache control for subresources and leverages it in the code where the editor loads a scene.

I've tested and will keep testing, but of course this is slippery ground, so let's be careful. 😀

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This PR is proudly donated by Pedrocorp.