CameraTextures like the ones in Unity

### Describe the project you are working on

A 2D game that needs a minimap, which will be a big zoomed-out camera.

### Describe the problem or limitation you are having in your project

Viewports are super hard to use and their use cases are limited since they depend on the nodetree. I want some symbols to appear on the minimap where the objects would be. With Unity I could make another camera, connect it to a CameraTexture, and use visibility layers. With Godot, I don't even know what I would do and I have been trying to do this for 2 hours.

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

Just a texture that mirrors a certain camera. The visibility layers for a 2d camera would mean it can see things that are in the same layers.

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

It would be in the list of textures in an exported variable (new texture > CameraTexture) and then you could set its camera to one in the scene.

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

The function to get a texture from the camera is slow to call every frame.

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

It's super tiny, intuitive, and beats the already built-in solution.