OpenGL `stroke_width` is a 2D array, whereas it is a float in cairo.

Nobody has claimed this yet.

## Description of bug / unexpected behavior

The following code is an example of a scene someone might run, with a direct modification to the `stroke_width` parameter.
```py
class a(Scene):
def construct(self):
number_plane = NumberPlane()
number_plane.stroke_width = 4

self.play(FadeIn(number_plane))
self.interactive_embed()
```

This would not work in `OpenGL` since `stroke_width` is a `_Data()` attribute and is a `2D` array.

`number_plane.stroke_width = [[4]]`

This is an issue as sensible code might break under `renderer=opengl`.

Relatively simple fix, make `stroke_width` a decarator.

## Expected behavior

No difference between `opengl` and `cairo` mobs.

## Additional comments

Start by comparing how stroke_width is represented and assigned in the OpenGL and cairo renderers, using the NumberPlane example as the reproduction case. Confirm that assigning a float behaves consistently in both renderers, then verify the example no longer requires a 2D array for OpenGL.

Written by the indexing model from the issue text.