Add a AT_SHADOW_PASS built-in

[QbieShay]

Describe the project you are working on

Godot Engine

Describe the problem or limitation you are having in your project

Consolidating multiple issue with a single clear proposal with approved design.

Users seems to continuously bump into the issue that some of their shaders (xray, distortions, billboarding) influence shadow casting, and it's pretty clear that Godot doesn't have information to make assumptions about what the shader needs to do for the user, so the users should have access to the information themselves, so they may adapt their shader.

• https://forum.godotengine.org/t/3d-x-ray-shader-affecting-the-shadows/99733/5
• Allow users to provide a custom shadow shader #4443

What's worse now is that we have lots of people relying on checking the value in a matrix to check whether they are in the shadow pass or not - this is implicit behaviour that people are relying on and it's really really bad. We should provide something with more guarantees for the users so that it may not break in future releases due to some unrelated changes. We should limit as much as possible this kind of situation because it's a frustrating experience for users and a maintainance nightmare for maintainers.

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

From #4443 (comment)

We should offer a built-in variable AT_SHADOW_PASS to allow for mesh manipulation during shadow pass.

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

See above

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

No

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

It can't be a plugin - needs internals exposed.