OpenGL - Cairo discrepancy: AttributeError: 'OpenGLCamera' object has no attribute 'frame'

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## Description of bug / unexpected behavior

Even in a MovingCameraScene, there is no camera frame **when rendering with OpenGL.**

## Expected behavior

A camera frame object just like in Cairo should exist.

## How to reproduce the issue

Code for reproducing the problem

```py
class LinearPredictiveCoding(MovingCameraScene):
def construct(self):
self.camera.frame.save_state()
```

## Logs
Terminal output

```
╭─────────────────────────────── Traceback (most recent call last) ────────────────────────────────╮
│ Python\Python310\lib\site-packages\manim\cli\render\commands.py:103 in render │
│ │
│ 101 │ │ │ │ │ with tempconfig(config): │
│ 102 │ │ │ │ │ │ scene = SceneClass(renderer) │
│ ❱ 103 │ │ │ │ │ │ rerun = scene.render() │
│ 104 │ │ │ │ │ if rerun or config["write_all"]: │
│ 105 │ │ │ │ │ │ renderer.num_plays = 0 │
│ 106 │ │ │ │ │ │ continue │
│ │
│ Python\Python310\lib\site-packages\manim\scene\scene.py:222 in render │
│ │
│ 219 │ │ """ │
│ 220 │ │ self.setup() │
│ 221 │ │ try: │
│ ❱ 222 │ │ │ self.construct() │
│ 223 │ │ except EndSceneEarlyException: │
│ 224 │ │ │ pass │
│ 225 │ │ except RerunSceneException as e: │
│ │
│ some_file.py:7 in construct │
│ │
│ ❱ 7 │ │ self.camera.frame.save_state() │
╰──────────────────────────────────────────────────────────────────────────────────────────────────╯
AttributeError: 'OpenGLCamera' object has no attribute 'frame'
```

## Additional comments

I'm well aware that this might be a known not implemented feature, but I'm creating this bug so that it is documented.

Start by tracing MovingCameraScene and the OpenGLCamera entry points involved when self.camera.frame is accessed. Compare the OpenGL camera behavior with the Cairo camera implementation, then reproduce the example and verify that a camera frame is available and supports save_state() under OpenGL.

Written by the indexing model from the issue text.