Reestablish CPT hinge for master tables (#8111)

* Reestablish CPT hinge fof master tables

We (I) somehow lost a line or two in gmt_support.c.  Added a test as well so that we are warned if this tests fails again.  Output is like this:

* Create cpt_hinge.sh

Add test script cpt_hinge.sh.

* Update test PS

src/gmt_support.c

@@ -8959,6 +8959,7 @@ struct GMT_PALETTE *gmt_get_palette (struct GMT_CTRL *GMT, char *file, enum GMT_
 			return (P);
 		}
 
+		master = (file && file[0]) ? file : GMT->current.setting.cpt;	/* Set master CPT prefix */
 		P = GMT_Read_Data (GMT->parent, GMT_IS_PALETTE, GMT_IS_FILE, GMT_IS_NONE, GMT_READ_NORMAL|GMT_CPT_CONTINUOUS, NULL, master, NULL);
 		if (!P) return (P);		/* Error reading file. Return right away to avoid a segv in next line */
 		/* Stretch to fit the data range */

test/baseline/grdimage.dvc

@@ -1,5 +1,6 @@
 outs:
-- md5: 8219134ac2d69f2e4123365c18252cfa.dir
-  size: 8666527
-  nfiles: 30
+- md5: e0f5ec57ec18927bec8fc07d3334a8b5.dir
+  size: 8865922
+  nfiles: 31
   path: grdimage
+  hash: md5

test/grdimage/cpt_hinge.sh

@@ -0,0 +1,21 @@
+#!/usr/bin/env bash
+# Remote grid with a soft-hinge cpt with/without +h to enable and set it
+# Bottom panel: Polar CPT with activated soft-hinge at 0, so z = 0 is white
+# Middle panel: Polar CPT with activated soft-hinge at 2000, so z = 2000 is white
+# Top panel:    Polar CPT with deactivated soft-hinge, so z_mid = --1368.5 is white
+
+gmt begin cpt_hinge ps
+	# Pass CPT and activate soft hinge at 0
+	gmt grdimage @earth_relief_06m -RPT -Cpolar+h -B
+	gmt coast -W0.25p
+	gmt colorbar -DJRM -Bxa1000 -By+l"Hinge = 0"
+	# Create new CPT and activate soft hinge at 2000
+	gmt makecpt -Cpolar+h2000 -T-5766/3029 -H > t.cpt
+	gmt grdimage @earth_relief_06m -RPT -Ct.cpt -B -Y8c
+	gmt coast -W0.25p
+	gmt colorbar -DJRM -Ct.cpt -Bxa1000 -By+l"Hinge = 2000"
+	# Pass CPT and do not activate any hinge
+	gmt grdimage @earth_relief_06m -RPT -Cpolar -B -Y8c
+	gmt coast -W0.25p
+	gmt colorbar -DJRM -Bxa1000 -By+l"No hinge set"
+gmt end show