[macOS 26] Stuttering and very unstable frame process time with vsync disabled

### Tested versions

- Reproducible in Forward+/Mobile backends in 4.5 release.
- Not reproducible in Compatibility mode.

### System information

Godot v4.5.stable.mono - macOS Tahoe (26.0.0) - Multi-window, 1 monitor - Metal (Forward+) - integrated Apple M3 Pro (Apple9) - Apple M3 Pro (11 threads) - 18.00 GiB memory

### Issue description

Below graph shows FPS and Process time in a completely empty scene (only root `Node3D`)...
- on **macOS 26 Tahoe**
- as a **separate window** (not embedded)
- with **v-sync disabled**
- in **Forward+** backend

Compatibility backend works OK, enabled V-Sync makes it OK, the editor and embedded game view are OK.

... vs when running embedded (which I assume enables v-sync):

As you can see the process time is very unstable, and so are e.g. animations, camera movement etc., anything that depends on `deltaTime`, which itself is also very unstable.

Also, the editor says the game is running on Metal 3.2, but supposedly macOS 26 comes with new Metal 4. Not sure where that value comes from, though:
```
Godot Engine v4.5.stable.mono.official.876b29033 - https://godotengine.org
Metal 3.2 - Forward+ - Using Device #0: Apple - Apple M3 Pro (Apple9)
```

### Steps to reproduce

It's reproducible in a completely empty project. To visualize the issue better, you can add an animated cube and just watch it, the animation will stutter.

### Minimal reproduction project (MRP)

N/A

No source file, test, or entry point is named. Start with an empty project on macOS 26 and compare Forward+ and Compatibility with v-sync disabled, using the reported frame and process-time behavior. Done means separate-window Forward+ runs with stable process time and deltaTime under the reported conditions.

Written by the indexing model from the issue text.