Wipetower fix: temperature-changing command was sometimes missing after the toolchange

Cause: variable holding last issued temperature was not reset where it should have been
This should fix issue #2685

src/libslic3r/GCode/WipeTower.cpp

@@ -1242,6 +1242,8 @@ void WipeTower::generate(std::vector<std::vector<WipeTower::ToolChangeResult>> &
     for (auto& used : m_used_filament_length) // reset used filament stats
         used = 0.f;
 
+    m_old_temperature = -1; // reset last temperature written in the gcode
+
     std::vector<WipeTower::ToolChangeResult> layer_result;
 	for (auto layer : m_plan)
 	{