FSR 2.2 does not enhance image quality in VR / OpenXR

### Tested versions

Reproduced in 4.2 Stable, 4.2.1 Stable, and 4.3 Dev 4

### System information

Godot v4.2.1.stable - Windows 10.0.22631 - Vulkan (Forward+) - dedicated NVIDIA GeForce RTX 3060 (NVIDIA; 31.0.15.4617) - 11th Gen Intel(R) Core(TM) i7-11700K @ 3.60GHz (16 Threads) - HP Reverb G2 VR headset running through WMR and SteamVR

### Issue description

FSR 2.2 does not enhance the image when OpenXR is enabled.

With OpenXR enabled and FSR 2.2 enabled:

With OpenXR enabled and bilinear scaling:

With OpenXR disabled and FSR 2.2 enabled:

With OpenXR disabled and bilinear scaling:

As you can see, despite the viewport clearly rendering with a lower resolution than when OpenXR is enabled, it has a much higher quality image with FSR 2.2 than when OpenXR is enabled with FSR 2.2.

The difference between FSR 2.2 being enabled and disabled shows no improvement when OpenXR is enabled, while it shows a clear improvement when OpenXR is disabled.

The difference is not due to the motion of the VR headset as that motion is retained when OpenXR is disabled on the viewport.

### Steps to reproduce

Enable FSR 2.2 in a project using OpenXR. Compare `get_viewport().use_xr = true` with `get_viewport().use_xr = false`.

### Minimal reproduction project (MRP)

[FSR 2.2 Bug Reproduction.zip](https://github.com/godotengine/godot/files/14489980/FSR.2.2.Bug.Reproduction.zip)

Start with the linked minimal reproduction project and reproduce the comparison using FSR 2.2 with OpenXR enabled and disabled, including the get_viewport().use_xr settings described. Done means FSR 2.2 produces the expected image-quality improvement with OpenXR enabled, comparable to the non-OpenXR result.

Written by the indexing model from the issue text.