Design tab: studio lighting and body edges in the 3D view

Reported: once extruded, a part is hard to read; the lighting says
little about its shape. Both lights of the object shaders sit near the
camera, so the sides of a part come out in almost the same tone, and
nothing marks where one face ends and the next begins.

- The phong shader gains a studio lighting model, chosen by a new
  lighting_model uniform: a sky/ground hemisphere in world space (up
  faces cool and bright, down faces warm and dark), a key light from the
  upper left and a weak fill from the right, a plastic-like highlight,
  and a darker base with a faint sheen toward the silhouette so curved
  faces read as round. GLCanvas3D::set_studio_lighting() makes a canvas
  draw its objects with it whatever the realistic-view preferences; only
  the Design canvas turns it on. Every canvas sets the uniform on each
  use, 0 for the slicer's, so they render as before.
- Every B-rep edge of a body is drawn as a thin dark line over it, depth
  tested and pulled a few pixels toward the eye so it wins against the
  faces meeting at it and hides behind the faces in front. The seam of a
  closed surface and degenerate edges are left out
  (GeometryEngine::display_edges). Edges are sampled once per shape and
  kept across recomputes that leave a body unchanged; bodies faded by
  body focus get fainter edges, and a dress-up previewing its result
  alone hides them with the bodies.

Tests: display_edges gives a box its 12 edges at their lengths, a
cylinder its two round rims without the seam, a cone its base rim
without the seam or the apex.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QK4VgguuCAk2hZLWgcjJb9

src/libslic3r/CAD/GeometryEngine.cpp

@@ -17,6 +17,7 @@
 #include <TopTools_IndexedMapOfShape.hxx>
 #include <TopTools_ListOfShape.hxx>
 #include <TopTools_IndexedDataMapOfShapeListOfShape.hxx>
+#include <TopTools_ListIteratorOfListOfShape.hxx>
 #include <Poly_Triangulation.hxx>
 #include <gp_Ax2.hxx>
 #include <gp_Dir.hxx>
@@ -611,6 +612,27 @@ std::vector<Vec3d> GeometryEngine::sample_edge_world(const TopoDS_Edge& edge, do
     return pts;
 }
 
+std::vector<std::vector<Vec3d>> GeometryEngine::display_edges(const TopoDS_Shape& shape, double chord_tol)
+{
+    TopTools_IndexedDataMapOfShapeListOfShape faces_of_edge;
+    TopExp::MapShapesAndAncestors(shape, TopAbs_EDGE, TopAbs_FACE, faces_of_edge);
+    std::vector<std::vector<Vec3d>> out;
+    for (int i = 1; i <= faces_of_edge.Extent(); ++i) {
+        const TopoDS_Edge& edge = TopoDS::Edge(faces_of_edge.FindKey(i));
+        if (BRep_Tool::Degenerated(edge))
+            continue;
+        bool seam = false;
+        for (TopTools_ListIteratorOfListOfShape it(faces_of_edge.FindFromIndex(i)); it.More() && !seam; it.Next())
+            seam = BRep_Tool::IsClosed(edge, TopoDS::Face(it.Value()));
+        if (seam)
+            continue;
+        std::vector<Vec3d> pts = sample_edge_world(edge, chord_tol);
+        if (pts.size() >= 2)
+            out.push_back(std::move(pts));
+    }
+    return out;
+}
+
 Vec3d GeometryEngine::face_centroid_world(const TopoDS_Face& face)
 {
     GProp_GProps props;