Complete the rib wipe tower port in WipeTower2

The rib tower is now always square (prime_tower_width is ignored, as the
GUI already implies), carries the rib origin offset like the BBL tower so
the rib tips sit inside the configured position, clamps the rib length to
the tower diagonal, and extends the ribs for short towers.

src/libslic3r/GCode/WipeTower2.hpp

@@ -69,9 +69,9 @@ public:
         const float brim = m_wipe_tower_brim_width_real;
         return BoundingBoxf(Vec2d(-brim, -brim), Vec2d(double(m_wipe_tower_width) + brim, double(m_wipe_tower_depth) + brim));
     }
-    // WT2 doesn't currently compute a rib-origin compensation like WipeTower (m_rib_offset),
-    // so expose a zero offset for consistency purposes (to maintain API parity).
-    Vec2f get_rib_offset() const { return Vec2f::Zero(); }
+    // Tower-local shift that puts the rib wall's first-layer min corner at the configured
+    // tower position, like WipeTower::get_rib_offset(). Zero unless the rib wall is used.
+    Vec2f get_rib_offset() const { return m_rib_offset; }
     float get_rib_width() const { return m_rib_width; }
     float get_rib_length() const { return m_rib_length; }
 
@@ -231,6 +231,7 @@ private:
     float  m_rib_width                    = 10;
     float  m_extra_rib_length             = 0;
     float  m_rib_length                   = 0;
+    Vec2f  m_rib_offset                   = Vec2f::Zero();
 
     bool   m_enable_arc_fitting           = false;
 
@@ -372,6 +373,10 @@ private:
 		float spacing);
 
     Polygon generate_rib_polygon(const WipeTower::box_coordinates& wt_box);
+
+    // Computes the depth reserved for a toolchange (shared by plan_toolchange() and the
+    // rib-wall square-tower replanning in generate()).
+    WipeTowerInfo::ToolChange set_toolchange(size_t old_tool, size_t new_tool, float layer_height, float wipe_volume, bool first_layer_plan);
 };