Add renderer settings dialog with tab categories per delegate

Surfaces each render delegate's settings (FLAG/INT/FLOAT/STRING) in a
per-viewport floating dialog opened from the gear popup. Settings are grouped
into tabs by key namespace prefix (split at ':'); unprefixed keys go into
General. Changes apply immediately; Reset Defaults restores defValues.
Settings list is cached per delegate to avoid TF_WARN flooding the log on
every frame.

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

src/core/UsdSceneRenderer.h

@@ -1,4 +1,4 @@
-#pragma once
+#pragma once
 
 #include <glad/gl.h>
 
@@ -6,6 +6,8 @@
 #include <pxr/usd/usd/timeCode.h>
 #include <pxr/imaging/glf/drawTarget.h>
 #include <pxr/usdImaging/usdImagingGL/engine.h>
+#include <pxr/usdImaging/usdImagingGL/rendererSettings.h>
+#include <pxr/base/vt/value.h>
 #include <pxr/imaging/glf/simpleLight.h>
 #include <pxr/imaging/glf/simpleMaterial.h>
 #include <pxr/base/gf/vec3f.h>
@@ -187,6 +189,11 @@ public:
     pxr::TfTokenVector  GetRendererAovs()       const;
     bool                SetCurrentAov(const pxr::TfToken& aov);
 
+    pxr::UsdImagingGLRendererSettingsList GetRendererSettingsList() const;
+    pxr::VtValue    GetRendererSetting(const pxr::TfToken& id) const;
+    void            SetRendererSetting(const pxr::TfToken& id,
+                                       const pxr::VtValue& value);
+
     bool              GetAAEnabled()            const { return m_aaEnabled; }
     void              SetAAEnabled(bool enabled)      { m_aaEnabled = enabled; }