Movie writer inconsistent deltas at start set movie writer fps to 30

### Tested versions

- Reproducible 4.3.stable & 4.2.1.stable

### System information

Godot v4.3.stable.mono - Windows 10.0.22631 - Vulkan (Forward+) - integrated Intel(R) UHD Graphics 630 (Intel Corporation; 31.0.101.2127) - Intel(R) Core(TM) i7-8850H CPU @ 2.60GHz (12 Threads). Also windows 11 desktop with AMD 7950x and RTX 3090. Vaguely remember trying on old linux box too.

### Issue description

The first few frames have differing deltas when in movie mode set to 30 fps. If I set physics ticks to 30 then deltas are constant.

### Steps to reproduce

Create 2D or 3D scene and add this script:

```
var previousDelta:float = 0
# Called every frame. 'delta' is the elapsed time since the previous frame.
func _process(delta: float) -> void:
if (previousDelta != 0 && previousDelta != delta):
push_error('delta is different')
previousDelta = delta
```

Set project settings movie writer fps to 30. Turn on movie maker mode and play and the error will trigger a few times at start then disappear.

### Minimal reproduction project (MRP)

[deltaproblem.zip](https://github.com/user-attachments/files/17317496/deltaproblem.zip)

Start with the attached minimal reproduction project and reproduce the initial differing deltas with movie maker mode set to 30 FPS. Trace the movie writer and frame-timing startup path to identify why the first few frames differ; done means the reproduction no longer reports errors at startup while preserving constant 30 FPS timing.

Written by the indexing model from the issue text.