[4.0] Add 2D scale factor property

[Ansraer]

4.0 version of #52137. Bugsquad edit: Rebase of #21446

This can be used to make 2D elements larger or smaller, independently of the current stretch mode.
Only the disabled and canvas stretch modes support this new property.

As with the 3.x version this doesn't solve the image scaling problem (again, we would need godotengine/godot-proposals#2924). In addition, font rendering currently looks weird when the scale factor is set to anything other than. After far too much time spent debugging I can now confirm that this is NOT caused by these changes. It would appear that font oversampling is currently broken. For more information please see #47957. Looks fine as of 17/12/2021 on linux. Guess someone already fixed font oversampling.

2d-scaling-demo4.zip

Pinging @Calinou, since he wanted to port this to 4.0 himself.

[mhilbrunner]

This has failing CI checks.

[akien-mga]

The 3.x PR has been merged, but this one still doesn't pass CI.

[Ansraer]

@akien-mga Yeah, I am really sorry about that. I have been really busy with university for the last few months right now and didn't have the time to dig deeper into the testcase that fails.
Hopefully I will have some time over the winter break between preparing for my exams. IIRC the problem is that a function that gets a screen pos from a character keeps returning (0,0) (or was it the other way around?).
I probably need to dig deeper into text rendering to figure this out. When I have the spare time I will probably also need to ping bruvzg about #47957. That bug really needs to be fixed before 4.0.

[Ansraer]

@akien-mga Should now pass all checks. Again, I am sorry that I didn't have time for this sooner.

[akien-mga]

Thanks!