Add support for Multiple Render Targets (MRT)

[samdze]

Describe the project you are working on:
Top-down 2D rpg with heavy use of visual effects, post-processing and custom depth tests.

Describe the problem or limitation you are having in your project:
Right now, I can't fill several buffers when an object is drawn by Godot (I also need to progressively read some of these same buffers to perform depth tests and the like, multi-pass shaders needed) unless I duplicate each object in different Viewport, which is extremely inconvenient and a waste of resources.

The ViewportContainer -> Viewport -> World approach is also very tedious to just setup a render target, and it doesn't allow spread out nodes to render to it.

So, a more flexible and unrestricted way of assigning render targets would be great.

I know MRT is scheduled for 4.0 as @reduz said here, I hope it can be added as soon as possible as I need to prototype the related features in the game.
I know that customizable render pipelines are scheduled too, together with MRT the possibilities will be very extensive.

Describe the feature / enhancement and how it helps to overcome the problem or limitation:
MRT is a very useful feature of modern GPUs and every major engine in the market supports it.
It is useful to create nice effects and post-processing in 3D and even in 2D when more than one buffer needs to be written at once by each object.

I think the rendering code rewrite in Vulkan is the perfect opportunity to add this feature, together with #496

Describe how your proposal will work, with code, pseudocode, mockups, and/or diagrams:
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If this enhancement will not be used often, can it be worked around with a few lines of script?:
No, severe duplication of nodes would be needed and there would be many inconveniences.

Is there a reason why this should be core and not an add-on in the asset library?:
This would be extremely inefficient to do in a way that is not core related.

[reduz]

This is planned for 4.0, still debating myself how to do it.

[samdze]

Thanks! Would also like to highlight this point for the proposal:

The ViewportContainer -> Viewport -> World approach is also very tedious to just setup a render target, and it doesn't allow spread out nodes to render to it.

So, a more flexible and unrestricted way of assigning render targets would be great.

[Zireael07]

I ran into the limitation of screen reading shaders, namely that they happen before alpha pass. This would likely help a lot in such cases!

[huyi7]

Been checking back from time to time hoping MRT can be implemented in Godot 4. That would enable us to do more efficiently GPGPU processing.

[clayjohn]

@huyi7 Check out #7916 as this is more likely the approach we would go with for MRTs