Movie : -JG +w option

Hi,

I'm wrapping up the sea-ice concentration movie for @Esteban82 and I *sometimes* got an error.
The script can't find a file (absolute path given)
```
projection="-JG0/0/10c+w25"
gmt grdimage ${DIR_wrk}/inset_illu.nc -Cinset_illu.cpt -t25 $projection -Rg
```

```
grdmath (gmtapi_export_grid): Could not find file [ /inset_illu.nc]
[Session gmt (0)]: Error returned from GMT API: GMT_GRID_WRITE_ERROR (19)
```

But sometimes it works.