Vulkan: DirectionalLight3D or SpotLight3D with Shadow Blur set to 0.0 exhibits acne, even at default bias

[Calinou]

Godot version

4.0.beta7

System information

Fedora 36, Vulkan Forward Plus, AMD Radeon RX 6900 XT

Issue description

DirectionalLight3D or SpotLight3D with Shadow Blur set to 0.0 exhibits acne, even at default bias. This doesn't seem to affect OmniLight3D, even if its bias is manually decreased to 0.03 (same value as SpotLight3D).

This may be a regression from #68339. cc @clayjohn
This may be fixable by not decreasing bias below a certain blur value (such as 0.1 or even 0.5).

I can reproduce this in both Forward Plus and Forward Mobile rendering methods.

Shadow Blur = 0.1	Shadow Blur = 0.0

Steps to reproduce

• Add a DirectionalLight3D or Spotlight3D node with shadows enabled.
• Set Shadow Blur to 0.0.

Minimal reproduction project

test_shadow_bias_blur.zip