Error in center of rotation for add_fixed_orientation_mobjects()

### Error in center of rotation for `add_fixed_orientation_mobjects()`
The calculated center for fixed orientation mobjects is incorrect for text strings. Specifically, the text's center of rotation is placed on the left edge of the text, instead of the center given from `.get_center()`.

**Code**:

```py
from manim import *

class Start(ThreeDScene):

def construct(self):

self.add(ThreeDAxes())
text = Tex("Some Long Text")
self.add_fixed_orientation_mobjects(text)
self.interactive_embed()
```

## Images of Incorrect rotation:

Images

## Additional comments
Ran using the OpenGL renderer on Mac

Reproduce the issue with the provided ThreeDScene example using the OpenGL renderer and inspect add_fixed_orientation_mobjects() with a Tex object. Compare the rotation center with text.get_center(); done means the text rotates around its actual center rather than its left edge.

Written by the indexing model from the issue text.