Subtractive Blending doesn't work on Compatibility Renderer

### Tested versions

I've tested various versions of Godot 4 going all the way back to when the Compatibility Renderer was first added. None of them have correctly rendered subtractive blending. It should be reproducible in any version of Godot 4 with the Compatibility Renderer.

### System information

Godot v4.4.dev5 - Windows 10.0.19045 - Multi-window, 3 monitors - OpenGL 3 (Compatibility) - NVIDIA GeForce RTX 3070 (NVIDIA; 32.0.15.6094) - AMD Ryzen 7 5800X 8-Core Processor (16 threads)

### Issue description

Subtractive Blended Materials show a solid color, instead of blending with whatever is drawn behind. This isn't an issue for Additive or Multiplicative Blended Materials.

In editor, it shows one of your editor theme colors:


In game, it seems to show a solid gray:


### Steps to reproduce

1. Create a project, set to Compatibility Renderer
2. Open a 3D Scene
3. Add a Camera3D
4. Add a MeshInstance3D, with a PlaneMesh rotated towards the Camera
5. Add a StandardMaterial3D to the MeshInstance3D, Shading Mode set to Unshaded, Blend Mode set to Subtract

### Minimal reproduction project (MRP)

N/A

Start by reproducing the issue with the listed Compatibility Renderer steps and compare subtractive behavior with additive and multiplicative blending. Trace the Compatibility Renderer path for StandardMaterial3D blend modes; done means subtractive materials blend with the content behind them in both the editor and a running game.

Written by the indexing model from the issue text.