Vulkan: DirectionalLight3D PSSM 2 Split Blending blends 2nd & non-existant 3rd split

[mrjustaguy]

Godot version

v4.0.dev.custom_build [89cf449fe]

System information

Windows 10, Vulkan, Nvidia Quadro K2000 461.92

Issue description

Blending is broken when using 2 splits instead of 4, as it treats the shadowmap as if it were 4 splits and not 2, resulting in the last split blending with non-existant splits that causes a Fade out effect, not related to the Fade Start.

Steps to reproduce

Create Directional light and Enable shadows and make a shadow
Go near the end of Max Distance, with Fade start at 1
Toggle Blend Splits on and off (On will result in a Fade, Off will result in expected result.)

Minimal reproduction project

No response

[Calinou]

• Related to Vulkan: DirectionalLight3D PSSM 2 Splits exhibits shadow acne not present in other modes, and shadows are unstable #59232.

[Calinou]

Still reproducible on 4.2.dev 6916349 (Linux, NVIDIA 535.113.01). This does not occur when using Compatibility, only Forward+ and Mobile.