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.cpp

@@ -369,6 +369,23 @@ bool UsdSceneRenderer::SetCurrentAov(const pxr::TfToken& aov)
     return false;
 }
 
+pxr::UsdImagingGLRendererSettingsList UsdSceneRenderer::GetRendererSettingsList() const
+{
+    if (!m_renderer) return {};
+    return m_renderer->GetRendererSettingsList();
+}
+
+pxr::VtValue UsdSceneRenderer::GetRendererSetting(const pxr::TfToken& id) const
+{
+    if (!m_renderer) return {};
+    return m_renderer->GetRendererSetting(id);
+}
+
+void UsdSceneRenderer::SetRendererSetting(const pxr::TfToken& id, const pxr::VtValue& v)
+{
+    if (m_renderer) m_renderer->SetRendererSetting(id, v);
+}
+
 // ===========================================================================
 // Camera State
 // ===========================================================================