Vulkan: GeometryInstance3D Transparency does not disable shadow rendering at high values

[Calinou]

Related to #55366 and godotengine/godot-proposals#3276.

Godot version

4.0.dev (5efe80f)

System information

Fedora 34, GeForce GTX 1080 (NVIDIA 470.74)

Issue description

GeometryInstance3D Transparency does not disable shadow rendering at high values. This affects both directional and point light shadows.

While we can't have semi-transparent shadows until godotengine/godot-proposals#3276 is implemented, we should probably disable shadow rendering after a certain threshold is reached (probably 0.5, but possibly higher).

transparency-shadow-rendering.mp4

Steps to reproduce

• Add a MeshInstance3D node (that will be faded) and a second MeshInstance3D to act as a floor.
• Add a DirectionalLight3D and SpotLight3D node, both with shadows enabled. Position them so they create visible shadows.
• Increase the Transparency property in the MeshInstance3D node up to 1.
• Notice how shadows remain fully visible.

Minimal reproduction project

test_geometry_transparency_shadow.zip

[clayjohn]

Shadows properly update for DirectionalLight3D now, but shadows still don't update for the SpotLight3D.