[3.x] Fix to allow TextureArrays to be serialized

[Ithamar]

This fixes serialisation of TextureArray based classes. For a long time (at least since 3.2) this has been broken, writing nulls as layer images in the resulting resource files.

This is targeting 3.x since serialisation is completely missing from the master branch, and code has diverged so much that there is no obvious way to fix it for both codebases in one go.

This will fix #54202, #38870, #34312 and many others.

NOTE: this commit was taken from V-Sekai@2ba3561, authored by @SaracenOne that I was made aware of by @Calinou in issue #54202

Tested using the following snippet:

	var i := Image.new()
        i.create(1, 1, false, Image.FORMAT_RGB8)
	i.lock()
	i.set_pixel(0,0, Color(1,0,0))
	i.unlock()
	ResourceSaver.save("res://image.tres", i)
	var l := TextureArray.new()
	l.create(1, 1, 1, Image.FORMAT_RGB8)
	l.set_layer_data(i, 1)
	ResourceSaver.save("res://layered.tres", l)

and checking the layered.tres for non-null layer data.

[SaracenOne]

I forgot about this. So yeah, for people still on 3.x, we might as well merge this I think

[akien-mga]

Is this really the right solution? RESOURCE_NOT_PERSISTENT seems like it's intended for resources, and here it's a dictionary. Indeed, the serialization code had to be changed to handle this dictionary.

Wouldn't this work with PROPERTY_USAGE_INTERNAL?

[akien-mga]

CC @Ithamar @KoBeWi