Add ability to set material and shadow blend modes separate from one another

[catboy-catfish]

I just had this idea for creating realistic(ish) transparent surfaces with transparent shadows.
The idea is that we use normal alpha blending for the surface itself, but use hashing and/or dithering for its shadows, however I don't think I can pull this off in Godot 4.21 Stable right now. I could just make a fork and try to implement this myself, but I'm not smart yet.

Right now I have this custom glass shader I did which uses Depth Prepass for the shadows. The surface looks great. The shadows look fine but not as good as they could be.

Best I can use is Hash blending, which does the shadows beautifully but it makes the surface look terrible.

TAA can reduce the grain/artifacts, but not completely so. It also creates new ghosting artifacts due to its temporal nature.

This is my feature request: add an ability to set the shadow's blending mode separately from the surface's. Blender EEVEE is capable of doing this, and I've heard Unity takes a similar approach. Right now I'm using version 4.2.1 Stable and I'll likely stick to that until a stable version of something like 4.3 is released. Maybe you could add this to 4.3 stable or in the meantime.

Thank you

[Calinou]

• Related to Implement dithered shadows for semi-transparent objects #3276.

This is already possible by using two MeshInstance3D nodes with the same mesh, at the same position:

• One that has its Cast Shadows property set to Off (the display mesh).
• One that has its Cast Shadow property set to Shadows Only (the shadow mesh).

In the shadow pass (AT_SHADOW_PASS), we could have alpha hashing be used when GeometryInstance3D Transparency property is used when this value is greater than 0.0, since this doesn't disable shadow casting right now. In comparison, setting the transparency mode to Alpha will always disable shadow casting, so you can't handle AT_SHADOw_PASS in the shader.

Maybe you could add this to 4.3 stable or in the meantime.

4.3 is nearing feature freeze, so any new features that need a significant amount of work like this will be for 4.4 at the earliest.