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/core/UsdSceneRenderer.h

@@ -313,7 +313,7 @@ private:
 
     pxr::UsdStageRefPtr m_stage;
     std::shared_ptr<pxr::UsdImagingGLEngine> m_renderer;
-    pxr::TfToken    m_currentRendererPlugin; ///< Active plugin ID (empty = default)
+    pxr::TfToken    m_currentRendererPlugin; ///< empty = use Storm/GL heuristic on init
     pxr::GlfDrawTargetRefPtr                  m_drawTarget;
 
     pxr::UsdImagingGLRenderParams m_renderParams;