[NTG-context] Columns and itemize not showing up as expected

[Norbert Melzer]

The MWE at the end of this mail typesets something like this:

* a      * e
* b      * f
* c
* d

But I'd expect something like this:

* a      * d
* b      * e
* c      * f

How can I get expected output?

```tex
\starttext
  \startcolumns[n=2]
    \startitemize
      \item a
      \item b
      \item c
      \item d
      \item e
      \item f
    \stopitemize
  \stopcolumns
\stoptext
```
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