NEW:add custom-gcode before first layer

Change-Id: I168e3038cb7d6850a5d6c568f348fed8000a915a

src/libslic3r/GCode/GCodeProcessor.hpp

@@ -595,6 +595,7 @@ namespace Slic3r {
         ExtruderTemps m_extruder_temps;
         float m_extruded_last_z;
         float m_first_layer_height; // mm
+        float m_zero_layer_height; // mm
         bool m_processing_start_custom_gcode;
         unsigned int m_g1_line_id;
         unsigned int m_layer_id;