Multiple threads tutorial should use Image.load() as a useful example

[goatchurchprime]

Your Godot version:
3.2.3.stable

Issue description:
I had an issue with the when a 7Mb jpg file of a satellite image took 450ms in load_jpg_from_buffer() for godot to uncompress/decode. This froze everything up and caused network polling problems.
See godotengine/godot#20056 (comment)

I think I fixed it by putting my Image loader into a thread.

At the moment the multiple threads tutorial page (link below) uses a completely contrived example of incrementing a counter.

It would be more useful if the example instead applied to decoding a big binary object (like a jpg) as this is likely to be the first application a programmer is going to be forced to use threads in godot for. My implementation is here.

Then you can have a hint on the Image.load() documentation that this can hang the system if it's a big file, but there is a solution. I do not think that ResourceInteractiveLoader helps here because (a) it doesn't apply to downloaded files in the user:// directory, and (b) that interactive loader will still call the image loading function as a single frame-hanging blob.

URL to the documentation page (if already existing):
https://docs.godotengine.org/en/stable/tutorials/threads/using_multiple_threads.html

See also #2148