Fix of M106 on every new layer #7094

after parallelization of CoolingBuffer:
Remember the last fan speed emitted at the previous layer.

src/libslic3r/GCode/CoolingBuffer.hpp

@@ -41,6 +41,8 @@ private:
     // X,Y,Z,E,F
     std::vector<char>           m_axis;
     std::vector<float>          m_current_pos;
+    // Current known fan speed or -1 if not known yet.
+    int                         m_fan_speed;
     // Cached from GCodeWriter.
     // Printing extruder IDs, zero based.
     std::vector<unsigned int>   m_extruder_ids;