QuadMesh Shader technique in XR

### Tested versions

Godot 4.2.1, 4.2.2.rc2

### System information

Using Quest 3 + Sidequest

### Issue description

I am using a Quad Mesh placed in front of the camera with a shader material applied. It works correctly inside Godot preview editor and on a windows exe file, but when loading the game on quest, the mesh turns to black, obscuring my vision. I can tell it's the mesh causing the issue because 1. I can put my hands slightly through the mesh and see them 2. If I delete the mesh, it works, just without the shader.

### Steps to reproduce

1. Open XR
2. Quad Mesh in front of camera with shader material applied
3. Export to Quest
4. Launch

### Minimal reproduction project (MRP)

This would take a while, but the specific shader I'm using is in this zip file.
[psx_camera_shader.zip](https://github.com/godotengine/godot/files/14733879/psx_camera_shader.zip)

Start by reproducing the attached shader from psx_camera_shader.zip on a Quest 3 using the listed Godot versions, comparing the editor and Windows results with the Quest output. Confirm the Quad Mesh shader renders correctly in XR without turning black or obscuring the user's view.

Written by the indexing model from the issue text.