Make mbl.has_mesh() a method

Marlin/src/feature/bedlevel/bedlevel.cpp

 bool leveling_is_valid() {
   return
     #if ENABLED(MESH_BED_LEVELING)
-      mbl.has_mesh
+      mbl.has_mesh()
     #elif ENABLED(AUTO_BED_LEVELING_BILINEAR)
       !!bilinear_grid_spacing[X_AXIS]
     #elif ENABLED(AUTO_BED_LEVELING_UBL)

Marlin/src/feature/bedlevel/mbl/mesh_bed_leveling.cpp

 
   mesh_bed_leveling mbl;
 
-  bool mesh_bed_leveling::has_mesh;
-
   float mesh_bed_leveling::z_offset,
         mesh_bed_leveling::z_values[GRID_MAX_POINTS_X][GRID_MAX_POINTS_Y],
         mesh_bed_leveling::index_to_xpos[GRID_MAX_POINTS_X],
   }
 
   void mesh_bed_leveling::reset() {
-    has_mesh = false;
     z_offset = 0;
     ZERO(z_values);
   }

Marlin/src/feature/bedlevel/mbl/mesh_bed_leveling.h

 
 class mesh_bed_leveling {
 public:
-  static bool has_mesh;
   static float z_offset,
                z_values[GRID_MAX_POINTS_X][GRID_MAX_POINTS_Y],
                index_to_xpos[GRID_MAX_POINTS_X],
 
   static void reset();
 
+  FORCE_INLINE static bool has_mesh() {
+    for (uint8_t x = 0; x < GRID_MAX_POINTS_X; x++)
+      for (uint8_t y = 0; y < GRID_MAX_POINTS_Y; y++)
+        if (z_values[x][y]) return true;
+    return false;
+  }
+
   static void set_z(const int8_t px, const int8_t py, const float &z) { z_values[px][py] = z; }
 
   static inline void zigzag(const int8_t index, int8_t &px, int8_t &py) {

Marlin/src/gcode/bedlevel/mbl/G29.cpp

         SERIAL_PROTOCOLLNPGM("Mesh probing done.");
         BUZZ(100, 659);
         BUZZ(100, 698);
-        mbl.has_mesh = true;
 
         gcode.home_all_axes();
         set_bed_leveling_enabled(true);
         SERIAL_CHAR('Z'); echo_not_entered();
         return;
       }
-
-      mbl.has_mesh = true; // set since user manually entered a mesh point
-
       break;
 
     case MeshSetZOffset:

Marlin/src/module/configuration_store.cpp

  */
 
 // Change EEPROM version if the structure changes
-#define EEPROM_VERSION "V48"
+#define EEPROM_VERSION "V49"
 #define EEPROM_OFFSET 100
 
 // Check the integrity of data offsets.
   //
   // MESH_BED_LEVELING
   //
-  bool mbl_has_mesh;                                    // mbl.has_mesh
   float mbl_z_offset;                                   // mbl.z_offset
   uint8_t mesh_num_x, mesh_num_y;                       // GRID_MAX_POINTS_X, GRID_MAX_POINTS_Y
   #if ENABLED(MESH_BED_LEVELING)
 
   #if ENABLED(AUTO_BED_LEVELING_BILINEAR)
     refresh_bed_level();
-    //set_bed_leveling_enabled(leveling_is_on);
   #endif
 
   #if HAS_MOTOR_CURRENT_PWM
         "MBL Z array is the wrong size."
       );
       const uint8_t mesh_num_x = GRID_MAX_POINTS_X, mesh_num_y = GRID_MAX_POINTS_Y;
-      EEPROM_WRITE(mbl.has_mesh);
       EEPROM_WRITE(mbl.z_offset);
       EEPROM_WRITE(mesh_num_x);
       EEPROM_WRITE(mesh_num_y);
       EEPROM_WRITE(mbl.z_values);
     #else // For disabled MBL write a default mesh
-      const bool leveling_is_on = false;
       dummy = 0.0f;
       const uint8_t mesh_num_x = 3, mesh_num_y = 3;
-      EEPROM_WRITE(leveling_is_on);
       EEPROM_WRITE(dummy); // z_offset
       EEPROM_WRITE(mesh_num_x);
       EEPROM_WRITE(mesh_num_y);
       // Mesh (Manual) Bed Leveling
       //
 
-      bool leveling_is_on;
       uint8_t mesh_num_x, mesh_num_y;
-      EEPROM_READ_ALWAYS(leveling_is_on);
       EEPROM_READ(dummy);
       EEPROM_READ_ALWAYS(mesh_num_x);
       EEPROM_READ_ALWAYS(mesh_num_y);
 
       #if ENABLED(MESH_BED_LEVELING)
-        if (!validating) {
-          mbl.has_mesh = leveling_is_on;
-          mbl.z_offset = dummy;
-        }
+        if (!validating) mbl.z_offset = dummy;
         if (mesh_num_x == GRID_MAX_POINTS_X && mesh_num_y == GRID_MAX_POINTS_Y) {
           // EEPROM data fits the current mesh
           EEPROM_READ(mbl.z_values);