Fix of 8f293f0cb5

When was set use_external_mp_once to true then after first calling the avoid crossing perimeters this flag was reset which cases that on the second call of the avoid crossing perimeters the travel move didn't process as external.

src/libslic3r/GCode/AvoidCrossingPerimeters.hpp

@@ -18,6 +18,7 @@ public:
     // Routing around the objects vs. inside a single object.
     void        use_external_mp(bool use = true) { m_use_external_mp = use; };
     void        use_external_mp_once()  { m_use_external_mp_once = true; }
+    bool        used_external_mp_once() { return m_use_external_mp_once; }
     void        disable_once()          { m_disabled_once = true; }
     bool        disabled_once() const   { return m_disabled_once; }
     void        reset_once_modifiers()  { m_use_external_mp_once = false; m_disabled_once = false; }