Implement a square fill mode in GradientTexture2D

[MewPurPur]

Describe the project you are working on

2D Platformer

Describe the problem or limitation you are having in your project

I realized how useful this could be when I wanted to add a mild vignette effect around the screen. The radial fill mode is good, but it's too thick around the corners. So I would have to go to a sprite editor to draw my own texture.

Describe the feature / enhancement and how it helps to overcome the problem or limitation

A square gradient would solve this like so:

It can also be used for other effects, for example this retro-style checkpoint:

Or a grid tile:

Simple blob shadows, if you prefer a rectangular one for something (although it wouldn't look very nice without round corners):

Or just some other interesting textures.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

Already implemented in godotengine/godot#76151

This is an example of what it does when the difference between offset points is different in X and Y:

I am not using a rectangle because the texture would "break" if the FROM and TO points share a coordinate. Also, there is the brain illusion that makes you think diagonals are darker, and I wonder if I should use rounded corners to get rid of that. Right now, though, I think a simple implementation using squares is the best, and rounded corners could be a separate mode.

If this enhancement will not be used often, can it be worked around with a few lines of script?

For some cases a normal sprite editor suffices, even without gradients. But otherwise you need to spend a lot of time or use another tool.

Is there a reason why this should be core and not an add-on in the asset library?

Discoverability for the people who would make use of it.