GRIB: avoid quadratic performance on GRIB2 datasets using subgrids wi…

…thin a single GRIB message (to be opposed to several GRIB messages concatenated) (fixes OSGeo#2288)

gdal/frmts/grib/degrib/degrib/degrib2.cpp

@@ -1152,7 +1152,10 @@ int ReadGrib2Record (VSILFILE *fp, sChar f_unit, double **Grib_Data,
 
       /* Note we are getting data back either as a float or an int, but not
        * both, so we don't need to allocated room for both. */
-      unpk_g2ncep (&kfildo, (float *) (IS->iain), IS->iain, &(IS->nd2x3),
+      unpk_g2ncep (&kfildo,
+                   j == subgNum ? (float *) (IS->iain) : nullptr,
+                   j == subgNum ? IS->iain : nullptr,
+                  &(IS->nd2x3),
                   IS->idat, &(IS->nidat), IS->rdat, &(IS->nrdat), IS->is[0],
                   &(IS->ns[0]), IS->is[1], &(IS->ns[1]), IS->is[2],
                   &(IS->ns[2]), IS->is[3], &(IS->ns[3]), IS->is[4],

gdal/frmts/grib/degrib/g2clib/g2_getfld.c

@@ -439,7 +439,7 @@ g2int g2_getfld(unsigned char *cgrib,g2int cgrib_length, g2int ifldnum,g2int unp
         //   bitmap before the requested field.
         //
         if (isecnum == 6) {
-          if (unpack) {   // unpack bitmap
+          if (numfld==ifldnum && unpack) {   // unpack bitmap
             iofst=iofst-40;           // reset offset to beginning of section
             bmpsave=lgfld->bmap;      // save pointer to previous bitmap
             jerr=g2_unpack6(cgrib,cgrib_length,&iofst,lgfld->ngrdpts,&lgfld->ibmap,