ViewportTexture shader parameter breaks after project reload

### Tested versions

- Reproducible in v4.3.stable, v4.4.1.stable and v4.5.beta2

### System information

Godot v4.5.beta2 - Linux Mint 22.1 (Xia) on X11 - X11 display driver, Multi-window, 2 monitors - Vulkan (Forward+) - dedicated AMD Radeon RX 6800 XT (RADV NAVI21) - AMD Ryzen 5 3600 6-Core Processor (12 threads) - 31.28 GiB memory

### Issue description

Using a ViewportTexture as a shader parameter works until you reload the project, at which point it seems to be replaced with the broken purple pattern texture.

Working as intended:

After reload:

### Steps to reproduce

1. Create a SubViewport and put a Camera3D inside.
2. Create a quad MeshInstance3D and give it a new shader material with "Local to Scene" checked.
3. Add a shader script:
```
shader_type spatial;
uniform sampler2D ScreenTexture;
void vertex() { }
void fragment() {
ALBEDO = texture(ScreenTexture, UV).rgb;
}
```
4. Set the Screen Texture shader parameter to a ViewportTexture, choose the SubViewport.
5. Reload the project.

### Minimal reproduction project (MRP)

[viewporttexturetest.zip](https://github.com/user-attachments/files/21061379/viewporttexturetest.zip)
To reproduce you'll first need to clear and replace the shader's texture parameter with a new ViewportTexture.

Open the attached viewporttexturetest.zip minimal reproduction project and follow the listed steps with a SubViewport, Camera3D, quad MeshInstance3D, and shader material. First reproduce the broken purple texture after reloading the project; done means the ViewportTexture shader parameter remains valid after reload.

Written by the indexing model from the issue text.