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/slic3r/GUI/CAD/DesignCanvas.cpp

@@ -46,6 +46,7 @@ DesignCanvas::DesignCanvas(wxWindow* parent)
     // cannot be null. Borrow the plater's, as the editor canvases do.
     m_canvas->set_process(&wxGetApp().plater()->background_process());
     m_canvas->set_type(GLCanvas3D::ECanvasType::CanvasView3D);
+    m_canvas->set_studio_lighting(true);   // see GLCanvas3D::m_studio_lighting and phong.fs
 
     // CAD navigation, this canvas only: left-drag sweeps a selection rubber band, so orbit
     // moves to middle-drag and pan to right-drag. Design is a different modality from
@@ -1347,6 +1348,7 @@ void DesignCanvas::set_body_hidden(bool on)
 {
     if (m_body_hidden == on) return;
     m_body_hidden = on;
+    m_sketch_tool.set_body_edges_hidden(on);
     reload(true);   // hides/show base bodies + flips the ghost opaque/faint for preview-only mode
 }