Anti aliasing on second viewport causes white edges

### Tested versions

Lastest version.

### System information

Godot v4.2.1.stable - Windows 10.0.19045 - Vulkan (Forward+) - dedicated NVIDIA GeForce RTX 4070 Ti (NVIDIA; 31.0.15.5186) - AMD Ryzen 7 5800X3D 8-Core Processor (16 Threads)

### Issue description

When using a main viewport and a secondary to draw 3D models and enabling anti aliasing on the second one. The edges of the 3D models white-out on different angles of the light.

### Steps to reproduce

Create a SubViewport. Add 3d Model to a camera. Enable anti aliasing on the viewport.

https://imgur.com/tNIj7cX
https://i.imgur.com/mTdHzy8.png
https://i.imgur.com/QXZ6zho.png

### Minimal reproduction project (MRP)

[Test.zip](https://github.com/godotengine/godot/files/15017808/Test.zip)

Start by opening Test.zip and reproducing the issue with a main viewport and a SubViewport containing a 3D model, using anti-aliasing on the secondary viewport. Compare the Vulkan rendering at different light angles and trace the SubViewport anti-aliasing path. Done means the model edges no longer turn white under the reported conditions.

Written by the indexing model from the issue text.