Vulkan: PCSS-style shadow size options no longer implemented, yet still used in the code

[Calinou]

Godot version

4.0.dev (659178e)

System information

Fedora 34, GeForce GTX 1080 (NVIDIA 465.31)

Issue description

#45023 removed the PCSS-like features for directional, omni and spot shadows since they were too demanding. However, increasing Light Size or Light Angular Distance above 0 will still decrease shadow quality noticeably.

We should either reimplement this feature in a more efficient way, or remove the remnants of the old code entirely. I've tried my hand at removing the code here, but the shader no longer runs because the binding sizes no longer match. I don't know how to fix that.

See also #50088.

Steps to reproduce

• Add a DirectionalLight3D node and enable shadows.
• Set the DirectionalLight3D node's Angular Distance property to a value above 0, and notice how the shadow quality degrades. Yet, the sky shader will still use this value to enlarge the sun on the procedural sky.
• Add a OmniLight3D node and enable shadows.
• Set the OmniLight3D node's Shadow Size property a value above 0, and notice the shadow quality degrading.
• Do the same as OmniLight3D for SpotLight3D.

Minimal reproduction project

test_shadow_size.zip