Depth of mobjects doesn't get calculated correctly when `--renderer=opengl`

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## Description of bug / unexpected behavior

The depth doesnt get calculated correctly when `renderer` is set to `opengl`.
The mobjects seems to be added to the depthbuffer according to their order of addition in the scene instead of their position relative to the camera.

## Expected behavior

Expected Behaviour with `cairo` renderer:

https://github.com/user-attachments/assets/5d5e4dd1-2f79-471c-8035-b8c53973ec09

Actual Behaviour with `opengl` renderer:

https://github.com/user-attachments/assets/517a0382-0cbb-4088-b722-45602c667e08

## How to reproduce the issue

Code for reproducing the problem

```py
from manim import *
from svgelements import Path
from random import random
class m(ThreeDScene):
def construct(self) -> None:
cubes = VGroup(*[
Cube(fill_color=color, fill_opacity=1.0,stroke_width=1).move_to((0,0,-i*2))
for i,color in enumerate([RED, BLUE, GREEN, ORANGE])])
# self.camera.set_theta(-30*DEGREES)
axes = ThreeDAxes()

self.set_camera_orientation(PI/4, PI/4)
self.play(Write(axes))
self.play(Write(cubes), run_time = 5)
self.wait()
```

## Additional media files

Images/GIFs

## Logs
Terminal output

```
PASTE HERE OR PROVIDE LINK TO https://pastebin.com/ OR SIMILAR
```

## System specifications

System Details

- OS (with version, e.g., Windows 10 v2004 or macOS 10.15 (Catalina)):
- RAM:
- Python version (`python/py/python3 --version`):
- Installed modules (provide output from `pip list`):
```
PASTE HERE
```

LaTeX details

+ LaTeX distribution (e.g. TeX Live 2020):
+ Installed LaTeX packages:

## Additional comments

Run the supplied ThreeDScene reproduction with --renderer=opengl and compare it with the expected cairo output. Start at the OpenGL renderer's depth-buffer handling and trace how Cube positions and scene addition order affect depth. Done means the cubes are rendered according to their positions relative to the camera rather than their addition order.

Written by the indexing model from the issue text.