`TextMark` doesn't react to `staff-padding`

[Werner LEMBERG]

Please have a look at this snippet:

```
{
  \override Score.TextMark.staff-padding = 2
  \textMark "x" c''
  \textMark "g" c''
}
```

I expect that both the 'x' and the 'g' are aligned at the baseline.
However, it doesn't work.

AFAICS, the `TextMark` grob contains `side-position-interface`.  What
am I missing?


    Werner