Flush the wipe tower planner queue with M400 on Klipper

The wipe tower emitted G4 S0 to make the firmware finish its queued moves
before commands that must not take effect early. Klipper's G4 reads only the
P parameter, so that flush never happened there and a temperature change could
land seconds ahead of the moves it was meant to follow. Klipper now gets M400
instead, through one helper shared by both wipe tower implementations.

No change to any other firmware flavor's output, so no shipped profile or saved
project is affected.

src/libslic3r/GCode/WipeTower.cpp

@@ -617,6 +617,11 @@ Polygon generate_rectange_polygon(const Vec2f &wt_box_min ,const Vec2f & wt_box_
     return res;
 }
 
+const char* flush_planner_queue_command(GCodeFlavor flavor)
+{
+    return flavor == gcfKlipper ? "M400\n" : "G4 S0\n";
+}
+
 class WipeTowerWriter
 {
 public:
@@ -1190,7 +1195,7 @@ public:
 
 	WipeTowerWriter& flush_planner_queue()
 	{
-		m_gcode += "G4 S0\n";
+		m_gcode += flush_planner_queue_command(m_gcode_flavor);
 		return *this;
 	}
 

src/libslic3r/GCode/WipeTower.hpp

@@ -26,6 +26,14 @@ enum GCodeFlavor : unsigned char;
 Polylines construct_gap_for_skip_points(
     const Polygon& polygon, const std::vector<Vec2f>& skip_points, float wt_width, float gap_length, Polygon& insert_skip_polygon);
 
+// Returns the command that makes the firmware finish its queued moves, so a command or
+// custom-G-code boundary right after (resetting the extruder position, entering
+// [change_filament_gcode] / [filament_start_gcode]) is not reached early. Klipper acts on
+// such commands the moment it parses them, and its G4 reads only P, so the zero dwell the
+// other flavors use synchronizes nothing there — M400 does. Defined in WipeTower.cpp, shared
+// by WipeTower and WipeTower2.
+const char* flush_planner_queue_command(GCodeFlavor flavor);
+
 class WipeTower
 {
 public:

src/libslic3r/GCode/WipeTower2.cpp

@@ -386,7 +386,8 @@ public:
     }
 
     WipeTowerWriter2& switch_filament_monitoring(bool enable) {
-        m_gcode += std::string("G4 S0\n") + "M591 " + (enable ? "R" : "S0") + "\n";
+        m_gcode += flush_planner_queue_command(m_gcode_flavor);
+        m_gcode += std::string("M591 ") + (enable ? "R" : "S0") + "\n";
         return *this;
     }
 
@@ -625,7 +626,7 @@ public:
 	// Set extruder temperature, don't wait by default.
 	WipeTowerWriter2& set_extruder_temp(int temperature, bool wait = false)
 	{
-        m_gcode += "G4 S0\n"; // to flush planner queue
+        m_gcode += flush_planner_queue_command(m_gcode_flavor);
         m_gcode += "M" + std::to_string(wait ? 109 : 104) + " S" + std::to_string(temperature) + "\n";
         return *this;
     }
@@ -677,8 +678,8 @@ public:
     }
 
 	WipeTowerWriter2& flush_planner_queue()
-	{ 
-		m_gcode += "G4 S0\n"; 
+	{
+		m_gcode += flush_planner_queue_command(m_gcode_flavor);
 		return *this;
 	}