mtext truncates plot region in classic theme

In the classic theme, adding extra axis labels using `mtext` seems to truncate the plot region.

First `mtext` then `abline`:

```R
plt(0, 0, theme = "classic")
mtext("foo", at=0.3, side=2, line=2, las=1)
abline(h = 0.3)  # does NOT reach the y-axis
```

First `abline` then `mtext`:

```R
plt(0, 0, theme = "classic")
abline(h = 0.3)  # OK
mtext("foo", at=0.3, side=2, line=2, las=1)
```
