Vulkan: Alpha Antialiasing renders into depth prepass when it shouldn't, resulting in unexpected black pixels

[Calinou]

Godot version

4.2.dev2, 4.2.dev fe8a58b - Does not occur in 4.2.dev1 and prior

System information

Fedora 38, GeForce RTX 4090 (NVIDIA 535.104.05)

Issue description

Alpha Antialiasing renders into depth prepass when it shouldn't, resulting in unexpected black pixels:

Expected appearance, which is what you get with 4.2.dev1 and prior, or when disabling depth prepass in the Project Settings:

Antialiasing still renders correctly on the alpha scissor material's edges in this case.

For context, I'm looking to address #24069 for once and all 🙂

Steps to reproduce

Note: this can be reproduced regardless of whether MSAA is active in the Project Settings. With MSAA disabled, alpha antialiasing still helps a little, but not as much as with MSAA enabled.

• Create a MeshInsatnce with a material that uses the Alpha Scissor transparency mode.
• Set the alpha antialiasing mode to to Alpha Edge Blend or Alpha Edge Clip.
• Play around with the Alpha Antialiasing Edge value.

Minimal reproduction project

test_alpha_antialiasing.zip

[Calinou]

I bisected the regression to 7c812cd. cc @clayjohn