:class:`~.Unwrite` is affected by the direction of :class:`~.Write` previously applied to the text

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## Description of bug / unexpected behavior
Tested with the following code:

```py
class Test(Scene):
def construct(self):
a = Text("Hello").scale(3)
self.play(Write(a))
self.play(Unwrite(a))
```

results in:

https://user-images.githubusercontent.com/9102529/125365556-08b42780-e3a7-11eb-8c90-0e2fe7de53f5.mp4

but

```py
class Test(Scene):
def construct(self):
a = Text("Hello").scale(3)
self.play(Write(a), reverse=True)
self.play(Unwrite(a))
```

results in

https://user-images.githubusercontent.com/9102529/125365612-28e3e680-e3a7-11eb-880d-428d11201137.mp4

Changing the direction of `Write` changes the direction of `Unwrite`, which shouldn't be the case.

## Expected behavior

## How to reproduce the issue

Code for reproducing the problem

```py
Paste your code here.
```

## 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:

FFMPEG

Output of `ffmpeg -version`:

```
PASTE HERE
```

## Additional comments

Reproduce the behavior with the provided Scene example, comparing Unwrite after Write(a) and after Write(a, reverse=True). Start by reading the Write and Unwrite implementations; done means changing Write's direction no longer changes the direction used by the subsequent Unwrite animation.

Written by the indexing model from the issue text.