Do not let grdblend keep all the files open (#4976)

* Explore the open files in grdblen

Looks like we are opening files when we only need the header?

* Close open grid files after setting the offsets

* Delete center_cf_p.png

Author: PaulWessel

src/grdblend.c

@@ -190,6 +190,8 @@ GMT_LOCAL bool grdblend_overlap_check (struct GMT_CTRL *GMT, struct GRDBLEND_INF
 	return false;
 }
 
+EXTERN_MSC void gmtlib_close_grd (struct GMT_CTRL *GMT, struct GMT_GRID *G);
+
 GMT_LOCAL int grdblend_init_blend_job (struct GMT_CTRL *GMT, char **files, unsigned int n_files, struct GMT_GRID_HEADER **h_ptr, struct GRDBLEND_INFO **blend, bool delayed, struct GMT_GRID *Grid) {
 	/* Returns how many blend files or a negative error value if something went wrong */
 	int type, status, not_supported = 0, t_data, k_data = GMT_NOTSET;
@@ -532,7 +534,10 @@ GMT_LOCAL int grdblend_init_blend_job (struct GMT_CTRL *GMT, char **files, unsig
 		GMT_Report (GMT->parent, GMT_MSG_INFORMATION, "Blend file %s in %.12g/%.12g/%.12g/%.12g with %s weight %g [%d-%d]\n",
 			HHG->name, B[n].wesn[XLO], B[n].wesn[XHI], B[n].wesn[YLO], B[n].wesn[YHI], sense[B[n].invert], B[n].weight, B[n].out_j0, B[n].out_j1);
 
-		if (!B[n].memory && GMT_Destroy_Data (GMT->parent, &B[n].G)) return (-GMT_RUNTIME_ERROR);	/* Free grid unless it is a memory grid */
+		if (!B[n].memory) {	/* Free grid unless it is a memory grid */
+			gmtlib_close_grd (GMT, B[n].G);	/* Close the grid file so we don't have lots of them open */
+			if (GMT_Destroy_Data (GMT->parent, &B[n].G)) return (-GMT_RUNTIME_ERROR);
+		}
 	}
 
 	if (fabs (sub) > 0.0) {	/* Must undo shift earlier */