Fix:on Mac,step mesh shows a count of 0 initially

jira: STUDIO-9595
Change-Id: I9908d3eff394ada12dd9ab0a2c10ca8a78d4f1f6
(cherry picked from commit 8bafed6e0a0ac7237077e7e0a964706e35e5ed12)

src/slic3r/GUI/StepMeshDialog.hpp

@@ -43,6 +43,7 @@ private:
     wxStaticText* mesh_face_number_text;
     double m_last_linear = 0.003;
     double m_last_angle = 0.5;
+    unsigned int m_mesh_number = 0;
     std::future<unsigned int> task;
     bool validate_number_range(const wxString& value, double min, double max);
     void update_mesh_number_text();