NEW:add variable layer height

Change-Id: Idef7f0dea32e4faaeb4d6eb188695cc7b554099c
(cherry picked from commit 4969835159eebb356e9b26dab223ad0a18882b70)

src/slic3r/GUI/Gizmos/GLGizmosManager.hpp

@@ -256,6 +256,12 @@ public:
         else
             return nullptr;
     }
+    void* get_icon_texture_id(MENU_ICON_NAME icon) const{
+        if (icon_list.find((int)icon) != icon_list.end())
+            return icon_list.at(icon);
+        else
+            return nullptr;
+    }
 
     Vec3d get_flattening_normal() const;