Fix use of a non-integer index in split_path_inout.

This fixes warnings like:
```
/usr/lib64/python3.3/site-packages/matplotlib/bezier.py:294: DeprecationWarning: using a non-integer number instead of an integer will result in an error in the future
  path_out = Path(concat([verts_right, path.vertices[i:]]),
```

because `i += len(ctl_points) / 2` is a float. I assume that `ctl_points` is guaranteed to be divisible by two here.