Arrow shape fixed in case of even modeline height

Hi!
I have found small issue in get-arrow-dots function.
The result of calling ```(get-arrow-dots 'left 6 12)``` was
```
\".     \",
\"..    \",
\"...   \",
\"....  \",
\"..... \",
\"......\",
\"..... \",
\"....  \",
\"...   \",
\"..    \",
\".     \",
\"      \"
``` 
The problem is that this arrow is asymmetrical and it wasn't pixel perfect when rendered in modeline - that looked a bit ugly to me.

But from now on,   ```(get-arrow-dots 'left 6 12)```  generates the following:
```
\".     \",
\"..    \",
\"...   \",
\"....  \",
\"..... \",
\"......\",
\"......\",
\"..... \",
\"....  \",
\"...   \",
\"..    \",
\".     \"
``` 
Arrow is symmetrical now and looks perfectly.
Thank you.