Add global Preferences dialog and per-viewport render delegate

Preferences (Edit > Preferences, tabbed General/Animation/Viewport):
- Viewport tab: Default Render Delegate and Color Correction settings
- Persisted to %APPDATA%\UsdLayerManager\preferences.ini
- Render delegate: startup default only (not live-applied to running tiles)
- Color correction: global, applied live to all viewports on change

Render delegate per-viewport:
- Each tile toolbar switches its own delegate independently
- Saved per-tile in viewport_settings.ini; restored on next launch
- Tiles with no saved delegate get the pref default on startup

Color correction global:
- Removed from gear popup; lives in Preferences > Viewport only
- Applied to all tiles on init and on live change from Preferences

Fix InitRenderer always applying m_currentRendererPlugin when set,
rather than skipping when UsdImagingGLEngine auto-selects a default.

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

src/ui/Application.h

@@ -18,15 +18,25 @@
 
 namespace UsdLayerManager {
 
+/// Global (non-per-viewport) rendering preferences persisted to preferences.ini.
+struct AppPreferences {
+    std::string renderDelegate      = "HdStormRendererPlugin";
+    int         colorCorrectionMode = 1; ///< ColorCorrectionMode cast to int (1 = sRGB)
+    std::string ocioDisplay;
+    std::string ocioView;
+    std::string ocioColorSpace;
+    std::string ocioLook;
+};
+
 class Application {
 public:
     Application();
     ~Application();
-    
+
     bool Initialize(const std::string& windowTitle = "USD Layer Manager", int width = 1280, int height = 720);
     void Run();
     void Shutdown();
-    
+
 private:
     void Update();
     void RenderUI();
@@ -34,21 +44,27 @@ private:
     void RenderStatusBar();
     void RenderStageInfo();
     void RefreshManagers();
-    
+
     // File operations
     void OpenUsdFile();
-    void CreateNewUsdFile();       // creates fresh in-memory stage
+    void CreateNewUsdFile();
     void SaveUsdFile();
     void SaveUsdFileAs();
-    void CloseUsdFile();           // closes file-backed stage, falls back to default stage
+    void CloseUsdFile();
 
-    // Stage editing operations (also exposed via Stage menu)
+    // Stage editing operations
     void AddReferenceToStage();
     void CreatePrimOnStage(const std::string& typeName);
 
-    // Playblast — called each frame while a capture is in progress.
+    // Playblast
     void CapturePlayblastFrame();
-    
+
+    // Preferences
+    void LoadPreferences();
+    void SavePreferences();
+    void ApplyPrefsToAllViewports();
+    void RenderPreferencesDialog();
+
     std::unique_ptr<ImGuiContext> m_imguiContext;
     std::unique_ptr<IconManager> m_iconManager;
     std::unique_ptr<UsdStageManager> m_stageManager;
@@ -71,8 +87,18 @@ private:
     bool m_showCurveEditor;
     bool m_running;
 
-    MovieEncoder m_movieEncoder;
-    std::string  m_viewportSettingsPath; ///< %APPDATA%\UsdLayerManager\viewport_settings.ini
+    MovieEncoder   m_movieEncoder;
+    std::string    m_viewportSettingsPath;
+    std::string    m_prefsPath;
+    AppPreferences m_prefs;
+    bool           m_showPreferences = false;
+
+    // OCIO edit buffers for Preferences dialog
+    char m_prefOcioDisplayBuf[128]    = {};
+    char m_prefOcioViewBuf[128]       = {};
+    char m_prefOcioColorSpaceBuf[128] = {};
+    char m_prefOcioLookBuf[128]       = {};
+    bool m_prefOcioFieldsSynced       = false;
 };
 
 } // namespace UsdLayerManager