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/core/UsdSceneRenderer.h

@@ -43,6 +43,15 @@ enum class ShadingMode {
     Unlit,               ///< Smooth geometry, lighting disabled
 };
 
+/// Back-face culling style — maps directly to UsdImagingGLCullStyle.
+enum class CullStyle {
+    NoOpinion           = 0,
+    Nothing             = 1,
+    Back                = 2,
+    Front               = 3,
+    BackUnlessDoubleSided = 4,
+};
+
 /// Viewport color correction mode — maps to UsdImagingGLRenderParams::colorCorrectionMode.
 enum class ColorCorrectionMode {
     Disabled,    ///< No correction (raw linear output)
@@ -240,6 +249,9 @@ public:
     ShadingMode GetShadingMode()             const { return m_shadingMode; }
     void        SetShadingMode(ShadingMode m)     { m_shadingMode = m; m_forceRefresh = true; }
 
+    CullStyle   GetCullStyle()               const { return m_cullStyle; }
+    void        SetCullStyle(CullStyle c)          { m_cullStyle = c; m_forceRefresh = true; }
+
     ColorCorrectionMode GetColorCorrectionMode()              const { return m_colorCorrectionMode; }
     void                SetColorCorrectionMode(ColorCorrectionMode m) { m_colorCorrectionMode = m; m_forceRefresh = true; }
 
@@ -358,6 +370,7 @@ private:
 
     // Lighting settings (mirrors stageView.py viewSettings)
     ShadingMode          m_shadingMode;            // draw mode + lighting flags
+    CullStyle            m_cullStyle;              // back/front/none face culling
     ColorCorrectionMode  m_colorCorrectionMode;    // viewport color correction
     std::string          m_ocioDisplay;
     std::string          m_ocioView;