Rework how the 3D editor viewport's resolution scale is changed

[Calinou]

• Removed the Half Resolution per-viewport setting in favor of an editor setting that applies to all 3D viewports in all projects.
• Use an automatic rendering scale by default that's set to 100% on loDPI displays and 50% on hiDPI displays (editor scale >= 1.667).
• Add 25% and 33% scaling options in the Editor Settings. This is useful for low-end hardware with hiDPI displays (such as 4K laptops).

The rendering scale is updated when the editor setting changes, so no restart is required to apply rendering scale changes (as before).

This pull request can be remade for the 3.x branch if we agree on this one.

This closes godotengine/godot-proposals#2125.

[alexfreyre]

I agree with this PR but it looks that the main concern is about hiDPI displays,
I have a laptops that has 3000x2000 resolution and an Nvidia 940m. I use to connect an external FullHD monitor, and always I am facing that 1920x1080 is too much for my laptop with the viewport at 100% but 50% looks very bad.

Therefore I would like to have the 75% option, in my case there is no use case either 33% nor 25% because I set a low resolution in my system.

On the other hand I like a lot how Unreal has a 0-100% parameter that you can tweak according to your need.

Why we don't expose to the viewport the project-settings option "Shrink":
Under display/window/stretch/shrink

Or maybe a 0-100% viewport resolution scale

[Calinou]

Therefore I would like to have the 75% option, in my case there is no use case either 33% nor 25% because I set a low resolution in my system.

A 75% option requires linear filtering to look correct, and this option isn't available yet in Godot (it might be hackable by getting the ViewportTexture and enabling Texture::FLAG_FILTER on it). This can be implemented in a separate PR.