Vulkan: PCF shadow filtering not reimplemented yet

[Calinou]

Godot version: Git 11cd8b8

OS/device including version: Fedora 31, NVIDIA 440.44, GeForce GTX 1080

Issue description: 3D shadow map filtering hasn't been reimplemented yet, making shadows look "blocky". Some form of filtering needs to be implemented. Vulkan should be able to make this faster by making use of the texture gather extension (see this paragraph of the Tesseract rendering design document).

Steps to reproduce: Create a project and keep the default PCF5 shadow filtering (or try to switch to PCF13). Notice it has no effect, even after restarting the editor.

[Calinou]

This was made obsolete by #37678, closing.