Screen texture changes when an object with SSS is on screen and MSAA 3D is enabled

### Tested versions

Reproducible in : v4.2.1.stable.official [b09f793f5], v4.3.dev4.official [df78c0636]

### System information

Windows 10, RTX 2070 Max-Q, i7-9750H, Vulkan

### Issue description

The white cube has subsurface scattering enabled and the other cube has the shader at the bottom of the screen

This bug is baffling to me as the three required elements seem completely unrelated. I look forward to seeing discussion!

### Steps to reproduce

1. Enable MSAA
2. Create an object with a screen reading shader
3. Create an object with subsurface scattering
4. View the screen object and the SSS object together

### Minimal reproduction project (MRP)

[SSS_SCREEN_BUG.zip](https://github.com/godotengine/godot/files/14487620/SSS_SCREEN_BUG.zip)

Start with the attached SSS_SCREEN_BUG.zip minimal reproduction project and follow the four listed reproduction steps with MSAA enabled. Confirm the screen texture change when the screen-reading shader and subsurface-scattering object are visible together, then verify that the texture remains stable after the bug is fixed.

Written by the indexing model from the issue text.