Distant Shadow Splits have Wrong Bias

### Tested versions

since https://github.com/godotengine/godot/pull/68339

### System information

Vulkan

### Issue description

https://github.com/godotengine/godot/pull/48776 and https://github.com/godotengine/godot/pull/68339 Modify Blur and Bias respectively, but the latter adjusts bias based on the non modified blur value and doesn't take blur modification that the first PR is doing inside the shader.

The bias modification code should be applied to the modified - Final blur values, not the base blur.

### Steps to reproduce

Observe Distant Splits bias increase as blur levels increase, without blending splits, and observe how blur levels are the same for the splits, but the bias still increases for those distant splits

### Minimal reproduction project (MRP)

N/A

Start by tracing the distant shadow split blur and bias shader code described in PRs #48776 and #68339. Compare where the modified final blur and bias values are calculated, then reproduce the increasing bias across distant splits without split blending. Done means the bias uses the modified final blur value and no longer increases when split blur levels are equal.

Written by the indexing model from the issue text.