Port the skip-points gap wall to WipeTower2

prime_tower_skip_points was stubbed for Type2 towers: the wall call
hard-coded skip_points=false, the gap cutter received an empty vector,
and append_tcr2 never routed the entry travel. Now the toolchange entry
positions are precomputed from the finalized plan, the wall is cut open
at each entry, and the entry travel approaches around the tower bounding
box through the opening when it starts outside the tower. The geometry
helpers are re-synced with the BBL versions (add_extra_point guards,
per-point side selection). The cone wall keeps its separate path, where
the option stays inert.

Behavior change: non-BBL towers now honor the (default-on) checkbox with
gap walls and routed entries; with the option off the output is
unchanged, and the BBL tower path is untouched.

src/libslic3r/GCode/WipeTower2.hpp

@@ -233,6 +233,11 @@ private:
     float  m_extra_rib_length             = 0;
     float  m_rib_length                   = 0;
     Vec2f  m_rib_offset                   = Vec2f::Zero();
+    bool   m_use_gap_wall                 = false;
+    // Per plan layer, each toolchange's entry position (tower-local, un-shifted frame):
+    // where the wall is cut open so the entry travel does not cross the printed wall.
+    // Filled by compute_wall_skip_points() once the plan is final.
+    std::vector<std::vector<Vec2f>> m_wall_skip_points;
 
     bool   m_enable_arc_fitting           = false;
 
@@ -376,6 +381,8 @@ private:
 
     Polygon generate_rib_polygon(const WipeTower::box_coordinates& wt_box);
 
+    void compute_wall_skip_points();
+
     // 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);