Avoid collisions when moving Z down

Avoid collisions with previous extrusions in the same layer when moving Z down in an XYZ move.
This happens for example when starting a scarf joint after another perimeter was already printed.

Fixes SoftFever#7191

src/libslic3r/GCodeWriter.cpp

@@ -447,6 +447,12 @@ std::string GCodeWriter::travel_to_xyz(const Vec3d &point, const std::string &co
     // this function, fix it first.
     //std::terminate();
 
+    // Orca: If moving down during below the current layer nominal Z, force XY->Z moves to avoid collisions with previous extrusions
+    double nominal_z = m_pos(2) - m_lifted;
+    if (point(2) < nominal_z - EPSILON) { // EPSILON to avoid false matches due to rounding errors
+        this->set_current_position_clear(false); // This forces XYZ moves to be split into XY->Z
+    }
+
     /*  If target Z is lower than current Z but higher than nominal Z we
         don't perform the Z move but we only move in the XY plane and
         adjust the nominal Z by reducing the lift amount that will be