Add load support for GRIB2 GDT 12.

It seems there might be another ECMWF GRIB API bug for x1, y1, x2, y2 where they are treated as unsigned instead of signed. If that's the case (@dkillick - can you confirm?) then this is trickier to deal with than the float32 bug mentioned previously - the return value will be an `int` irrespective of whether the bug is fixed.

Perhaps we should use `grib_info -d` to locate the definition files and scan them for `unsigned[4] X1` vs. `signed[4] X1`? :scream: