Add per-viewport cull style option

Exposes UsdImagingGLCullStyle as a Cull Style sub-menu in the gear popup
(Back Unless Double-Sided / Back / Front / Nothing / No Opinion). Defaults
to BackUnlessDoubleSided, matching usdview. Setting persists per-viewport
to viewport_settings.ini.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ui/ViewportPanel.cpp

@@ -670,6 +670,7 @@ void ViewportPanel::SaveSettings(const std::string& path) const
         f << "AmbientLightOnly=" << (s.ambientLightOnly ? 1 : 0) << "\n";
         f << "DomeLightEnabled=" << (s.domeLightEnabled ? 1 : 0) << "\n";
         f << "ShadingMode="      << s.shadingMode        << "\n";
+        f << "CullStyle="        << s.cullStyle          << "\n";
     }
 }
 
@@ -741,6 +742,7 @@ void ViewportPanel::LoadSettings(const std::string& path)
             else if (key == "AmbientLightOnly") ts.ambientLightOnly = (val == "1");
             else if (key == "DomeLightEnabled") ts.domeLightEnabled = (val == "1");
             else if (key == "ShadingMode")      ts.shadingMode      = toInt(val, ts.shadingMode);
+            else if (key == "CullStyle")        ts.cullStyle        = toInt(val, ts.cullStyle);
         }
     }
 

src/ui/ViewportTile.cpp

@@ -106,6 +106,7 @@ ViewportTileSettings ViewportTile::GetSettings() const
     s.ambientLightOnly = m_renderer.GetAmbientLightOnly();
     s.domeLightEnabled = m_renderer.GetDomeLightEnabled();
     s.shadingMode      = static_cast<int>(m_renderer.GetShadingMode());
+    s.cullStyle        = static_cast<int>(m_renderer.GetCullStyle());
     return s;
 }
 
@@ -126,6 +127,7 @@ void ViewportTile::ApplySettings(const ViewportTileSettings& s)
     m_renderer.SetAmbientLightOnly(s.ambientLightOnly);
     m_renderer.SetDomeLightEnabled(s.domeLightEnabled);
     m_renderer.SetShadingMode(static_cast<ShadingMode>(s.shadingMode));
+    m_renderer.SetCullStyle(static_cast<CullStyle>(s.cullStyle));
 }
 
 // ---------------------------------------------------------------------------
@@ -1018,6 +1020,21 @@ void ViewportTile::RenderCompactToolbar(int tileIndex)
                 ImGui::EndMenu();
             }
 
+            // -- Cull Style --------------------------------------------------
+            if (ImGui::BeginMenu("Cull Style")) {
+                CullStyle curCull = m_renderer.GetCullStyle();
+                auto cullItem = [&](const char* label, CullStyle style) {
+                    if (ImGui::MenuItem(label, nullptr, curCull == style))
+                        m_renderer.SetCullStyle(style);
+                };
+                cullItem("Back Unless Double-Sided", CullStyle::BackUnlessDoubleSided);
+                cullItem("Back",                     CullStyle::Back);
+                cullItem("Front",                    CullStyle::Front);
+                cullItem("Nothing",                  CullStyle::Nothing);
+                cullItem("No Opinion",               CullStyle::NoOpinion);
+                ImGui::EndMenu();
+            }
+
             // -- AOV ---------------------------------------------------------
             {
                 pxr::TfTokenVector aovs = m_renderer.GetRendererAovs();

src/ui/ViewportTile.h

@@ -32,6 +32,7 @@ struct ViewportTileSettings {
     bool        ambientLightOnly = true;
     bool        domeLightEnabled = false;
     int         shadingMode      = 0;
+    int         cullStyle        = 4; // CullStyle::BackUnlessDoubleSided
     bool        showGuides       = false;
     bool        showProxy        = true;
     bool        showRender       = false;