AlphaMode::Mask keeps shadow on invisible objects

[MiniaczQ]

Problem

While AlphaMode::Opaque renders shadow as intended, AlphaMode::Blend renders shadow in full opacity as intended,
AlphaMode::Blend(f32) still renders shadow on all objects, despite them only being fully opaque or fully transparent.

Example

This is clearly visible in the transparency_3d example. (One of the spheres is masked away, yet still has a shadow)

[superdump]

This was covered in another issue but I don’t remember which it was at the moment.

[MiniaczQ]

I've checked 2 other issues which had to do with Mask and shadow casting, but I didn't see it pointed out, at least directly.
Those are:
#4576 - platform issue
#4372 - similar, but about the shape of the shadow instead

I guess the main difference is, if the whole object is masked away, there is no need to render it (or the shadow) whatsoever.
Technically, fixing #4372 would fix this, but this would be more optimal?

[JonahPlusPlus]

How about creating a ShadowMode enum (this would be a solution to #4372 as well)? Like AlphaMode but for shadows. If #4372 gets solved by applying the alpha mask to shadow, it might make sense to also have a mode for blending shadows together like AlphaMode::Blend does with textures. They could all be in one enum, so: ShadowMode::Opaque, ShadowMode::Mask(f32), ShadowMode::Blend and ShadowMode::None.

Pros: More control
Cons: If there is a default, you would have to set it every time in the material (Default for corresponding AlphaMode could be provided by making StandardMaterial.shadowMode be an Option<ShadowMode>, and using a default shadow mode if None)

[james7132]

This should be fixed now that we're using prepass shaders for shadows.