Inset within subplot panel fails

As reported on the [forum](https://forum.generic-mapping-tools.org/t/the-inset-picture-doesnt-displayed/2677), **inset** used in subplot panels fails in terms of placement. Clearly the -Dj setting is computing strange placement coordinates. In the original post, the BR box was plotted way to the right and later covered up by other plots.

A simple example contrasts plain inset versus inset inside a panel:

```
#!/bin/bash
# Shows problem with inset within subplot panel
gmt begin
gmt figure inset_in_panel png
gmt subplot begin 1x1 -Fs5c/4c -A
gmt subplot set 0
gmt coast -R70/138/13/56 -Glightgray -B
gmt inset begin -DjTL+w0.5c/0.8c+o0.1c -F+p1p,red
gmt inset end
gmt subplot end

gmt figure plain_inset png
gmt coast -R70/138/13/56 -Glightgray -B -JQ5c
gmt inset begin -DjTL+w0.5c/0.8c+o0.1c -F+p1p,red
gmt inset end

gmt end show
```

Only showing the failing inset_in_panel plot here (the other is fine):