G26: Allow to set retraction for UBL mesh test (#16511)

Marlin/Configuration.h

@@ -1234,6 +1234,7 @@
     #define MESH_TEST_HOTEND_TEMP  205    // (°C) Default nozzle temperature for the G26 Mesh Validation Tool.
     #define MESH_TEST_BED_TEMP      60    // (°C) Default bed temperature for the G26 Mesh Validation Tool.
     #define G26_XY_FEEDRATE         20    // (mm/s) Feedrate for XY Moves for the G26 Mesh Validation Tool.
+    #define G26_RETRACT_MULTIPLIER   1.0  // G26 Q (retraction) used by default between mesh test elements.
   #endif
 
 #endif

Marlin/src/gcode/bedlevel/G26.cpp

@@ -43,13 +43,16 @@
 #include "../../lcd/ultralcd.h"
 
 #define EXTRUSION_MULTIPLIER 1.0
-#define RETRACTION_MULTIPLIER 1.0
 #define PRIME_LENGTH 10.0
 #define OOZE_AMOUNT 0.3
 
 #define INTERSECTION_CIRCLE_RADIUS 5
 #define CROSSHAIRS_SIZE 3
 
+#ifndef G26_RETRACT_MULTIPLIER
+  #define G26_RETRACT_MULTIPLIER 1.0 // x 1mm
+#endif
+
 #ifndef G26_XY_FEEDRATE
   #define G26_XY_FEEDRATE (PLANNER_XY_FEEDRATE() / 3.0)
 #endif
@@ -509,7 +512,7 @@ void GcodeSuite::G26() {
   if (parser.seenval('T')) tool_change(parser.value_int());
 
   g26_extrusion_multiplier    = EXTRUSION_MULTIPLIER;
-  g26_retraction_multiplier   = RETRACTION_MULTIPLIER;
+  g26_retraction_multiplier   = G26_RETRACT_MULTIPLIER;
   g26_layer_height            = MESH_TEST_LAYER_HEIGHT;
   g26_prime_length            = PRIME_LENGTH;
   g26_bed_temp                = MESH_TEST_BED_TEMP;