Match onMeshUpdate() index sign (#15920)

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

 
       measured_z = 0;
 
-      xy_uint8_t meshCount;
+      xy_int8_t meshCount;
 
       // Outer loop is X with PROBE_Y_FIRST enabled
       // Outer loop is Y with PROBE_Y_FIRST disabled

Marlin/src/lcd/extensible_ui/lib/lulzbot/marlin_events.cpp

   }
 
   #if HAS_LEVELING && HAS_MESH
-    void onMeshUpdate(const uint8_t, const uint8_t, const float) {
+    void onMeshUpdate(const int8_t, const int8_t, const float) {
     }
   #endif
 }

Marlin/src/lcd/extensible_ui/ui_api.h

       float getMeshPoint(const xy_uint8_t &pos);
       void setMeshPoint(const xy_uint8_t &pos, const float zval);
       void onMeshUpdate(const uint8_t xpos, const uint8_t ypos, const float zval);
-      inline void onMeshUpdate(const xy_uint8_t &pos, const float zval) { onMeshUpdate(pos.x, pos.y, zval); }
+      inline void onMeshUpdate(const int8_t xpos, const int8_t ypos, const float zval) { onMeshUpdate((uint8_t)xpos, (uint8_t)ypos, zval); }
+      inline void onMeshUpdate(const xy_int8_t &pos, const float zval) { onMeshUpdate(pos.x, pos.y, zval); }
     #endif
   #endif