OpenGL scaling a `Vector` causes the line to get thinner

Nobody has claimed this yet.

Consider the following scene:

```python
class Test(Scene):
def construct(self):
v = Vector()
self.add(v)
self.wait()
self.play(v.animate.scale(2))
self.wait()
```

This is what happens when it is rendered with OpenGL:

https://user-images.githubusercontent.com/20936595/136078419-df31a01d-b8b8-42cf-9ebb-93836a04d0b2.mp4

Notice that while transforming, the line in the vector gets smaller and then grows again. This looks a little strange (I don't want it in any of my videos), and with Cairo, it doesn't do this.

Reproduce the provided Test scene with the OpenGL renderer and compare its Vector scaling animation with the Cairo result. Trace the Vector transformation and line rendering behavior involved in the animation; done means scaling no longer makes the line visibly thin and then grow again under OpenGL.

Written by the indexing model from the issue text.