Fix of Head crash on completion with sequential printing #5349

In addition to an existing "layer_z" placeholder variable,
a new "max_layer_z" has been added to custom G-code sections accumulating
maximum of "layer_z" over the print. This value may be used for example
in the end G-code of a sequential print to lift the print head
up before travel over the whole print bed.

src/libslic3r/GCode.hpp

@@ -351,6 +351,7 @@ private:
     // Support for G-Code Processor
     float                               m_last_height{ 0.0f };
     float                               m_last_layer_z{ 0.0f };
+    float                               m_max_layer_z{ 0.0f };
 #if ENABLE_GCODE_VIEWER_DATA_CHECKING
     double                              m_last_mm3_per_mm;
     float                               m_last_width{ 0.0f };