Change M421 to take coordinates

This makes `M421` more amenable for irregular matrices

Marlin/Marlin_main.cpp

  * M407 - Display measured filament diameter
  * M410 - Quickstop. Abort all the planned moves
  * M420 - Enable/Disable Mesh Leveling (with current values) S1=enable S0=disable
- * M421 - Set a single Z coordinate in the Mesh Leveling grid. X<index> Y<index> Z<offset in mm>
+ * M421 - Set a single Z coordinate in the Mesh Leveling grid. X<mm> Y<mm> Z<mm>
  * M500 - Store parameters in EEPROM
  * M501 - Read parameters from EEPROM (if you need reset them after you changed them temporarily).
  * M502 - Revert to the default "factory settings". You still need to store them in EEPROM afterwards if you want to.
    * M421: Set a single Mesh Bed Leveling Z coordinate
    */
   inline void gcode_M421() {
-    int x, y;
-    float z;
+    float x, y, z;
     bool err = false, hasX, hasY, hasZ;
-    if ((hasX = code_seen('X'))) x = code_value_short();
-    if ((hasY = code_seen('Y'))) y = code_value_short();
+    if ((hasX = code_seen('X'))) x = code_value();
+    if ((hasY = code_seen('Y'))) y = code_value();
     if ((hasZ = code_seen('Z'))) z = code_value();
 
     if (!hasX || !hasY || !hasZ) {
       SERIAL_ERROR_START;
-      SERIAL_ERRORLNPGM(MSG_ERR_XYZ_REQUIRED_FOR_M421);
+      SERIAL_ERRORLNPGM(MSG_ERR_M421_REQUIRES_XYZ);
       err = true;
     }
 
       err = true;
     }
 
-    if (!err) mbl.z_values[y][x] = z;
+    if (!err) mbl.set_z(select_x_index(x), select_y_index(y), z);
   }
 
 #endif

Marlin/configuration_store.cpp

 #include "configuration_store.h"
 
 #ifdef MESH_BED_LEVELING
-   #include "mesh_bed_leveling.h"
-#endif  // MESH_BED_LEVELING
+  #include "mesh_bed_leveling.h"
+#endif
 
 void _EEPROM_writeData(int &pos, uint8_t* value, uint8_t size) {
   uint8_t c;
     EEPROM_WRITE_VAR(i, mesh_num_x);
     EEPROM_WRITE_VAR(i, mesh_num_y);
     dummy = 0.0f;
-    for (int q=0; q<mesh_num_x*mesh_num_y; q++) {
-      EEPROM_WRITE_VAR(i, dummy);
-    }
+    for (int q=0; q<mesh_num_x*mesh_num_y; q++) EEPROM_WRITE_VAR(i, dummy);
   #endif // MESH_BED_LEVELING
 
   #ifndef ENABLE_AUTO_BED_LEVELING
     for (int y=0; y<MESH_NUM_Y_POINTS; y++) {
       for (int x=0; x<MESH_NUM_X_POINTS; x++) {
         CONFIG_ECHO_START;
-        SERIAL_ECHOPAIR("  M421 X", x);
-        SERIAL_ECHOPAIR(" Y", y);
+        SERIAL_ECHOPAIR("  M421 X", mbl.get_x(x));
+        SERIAL_ECHOPAIR(" Y", mbl.get_y(y));
         SERIAL_ECHOPAIR(" Z", mbl.z_values[y][x]);
         SERIAL_EOL;
       }

Marlin/configurator/config/language.h

 #define MSG_Z2_MAX                          "z2_max: "
 #define MSG_Z_PROBE                         "z_probe: "
 #define MSG_ERR_MATERIAL_INDEX              "M145 S<index> out of range (0-1)"
-#define MSG_ERR_XYZ_REQUIRED_FOR_M421       "M421 requires XYZ parameters"
+#define MSG_ERR_M421_REQUIRES_XYZ           "M421 requires XYZ parameters"
 #define MSG_ERR_MESH_INDEX_OOB              "Mesh XY index is out of bounds"
 #define MSG_M119_REPORT                     "Reporting endstop status"
 #define MSG_ENDSTOP_HIT                     "TRIGGERED"

Marlin/language.h

 #define MSG_Z2_MAX                          "z2_max: "
 #define MSG_Z_PROBE                         "z_probe: "
 #define MSG_ERR_MATERIAL_INDEX              "M145 S<index> out of range (0-1)"
-#define MSG_ERR_XYZ_REQUIRED_FOR_M421       "M421 requires XYZ parameters"
+#define MSG_ERR_M421_REQUIRES_XYZ           "M421 requires XYZ parameters"
 #define MSG_ERR_MESH_INDEX_OOB              "Mesh XY index is out of bounds"
 #define MSG_M119_REPORT                     "Reporting endstop status"
 #define MSG_ENDSTOP_HIT                     "TRIGGERED"