Vulkan: Directional light shadow rendering is broken on some splits after culling changes

[Calinou]

Godot version: Git 8e514bb

OS/device including version: Fedora 33, GeForce GTX 1080

Issue description: Directional light shadow rendering is broken on some splits after the recent culling changes. Also, the bias values look much larger than before even though the property defaults didn't change. You need to decrease them by a 10× factor to get the previous look again.

As you can notice on the video below, the fade is never visible even though the DirectionalLight's Fade Start property is at the default value (0.8).

I bisected the regression to 5485241.

Steps to reproduce:

• Add a DirectionalLight node.
• Enable shadows on the DirectionalLight.
• Play around with the DirectionalLight shadow's Max Distance property if you want to better see the effect.

Minimal reproduction project: test_directional_light_cull.zip

Current broken status

2021-01-11_23-51-38.mp4

Expected appearance

2021-01-12_00-51-05.mp4