Add a ModulateTexture type

[mich-gamedev]

Currently, a lot of theming options don't have good ways to modulate them, whether it be the expand arrows or check boxes on Trees, the icons on LineEdits, etc, they provide no color options in their themes unlike some other items. This makes multiple themes hard, because for such items if i want to have a unique color for them i need to make a new texture for each possible theme manually, and if i want to change that icon later i have to also change each duplicate manually.

It'd be a lot more convenient if, like how AtlasTexture can crop an existing texture, we had a ModulateTexture type which could modulate an existing texture, making changes to the original texture universal while still getting the benefits of unique colors.

While all my examples are UI, this isn't directly related to UI nodes which is why i put it in General Discussions.

Similar proposal i found, which wouldn't solve this issue but does show it could help others as well and solves the issue of accessibility/ease of use that there'd be with the ViewportTexture solution.
#9196

[WinnerWind]

I agree with this. It's really tedious to make lighter/darker versions of literally every single icon in my game and it's really proving to be a hassle.

I personally would love to just create a pure white icon for everything and modulate over that. That's how I do things in QML, a styling/arranging language i absolutely hate but I believe that modulating over icons is far superior.

Godot could also provide the option to "bake" these modulations but I'm not sure if that would be helpful.