Add per-viewport AOV selection (stageView.py GetRendererAovs port)

Exposes the render delegate's available AOVs in the gear popup under a new
"AOV" sub-menu. Selecting an AOV calls SetRendererAov on the engine and
persists the choice per-viewport in viewport_settings.ini. Switching the
render delegate resets the AOV to "color", matching stageView.py's
_handleRendererChanged. Menu is greyed-out when the delegate reports no AOVs.

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

src/core/UsdSceneRenderer.cpp

@@ -167,6 +167,7 @@ void main() { outColor = color; }
 UsdSceneRenderer::UsdSceneRenderer()
     : m_showGrid(true)
     , m_showCameraGuide(false)
+    , m_currentAov(pxr::TfToken("color"))
     , m_aaEnabled(false)
     , m_backgroundColor(0.15f, 0.15f, 0.15f)
     , m_shadingMode(ShadingMode::SmoothShaded)
@@ -296,8 +297,8 @@ void UsdSceneRenderer::InitRenderer() {
     }
     LOG_INFO("Selected renderer: " + std::string(m_renderer->GetCurrentRendererId().GetText()));
 
-    // Enable AOV "color" (matches stageView._handleRendererChanged)
-    m_renderer->SetRendererAov(pxr::TfToken("color"));
+    // Enable AOV (matches stageView._handleRendererChanged; uses saved m_currentAov)
+    m_renderer->SetRendererAov(m_currentAov);
 
     // Selection highlight color (usdview default: yellow)
     m_renderer->SetSelectionColor(pxr::GfVec4f(1.0f, 1.0f, 0.0f, 1.0f));
@@ -342,8 +343,9 @@ bool UsdSceneRenderer::SetRendererPlugin(const pxr::TfToken& pluginId) {
 
     bool ok = m_renderer->SetRendererPlugin(pluginId);
     if (ok) {
-        // Re-enable colour AOV after delegate switch (mirrors usdtweak behaviour)
-        m_renderer->SetRendererAov(pxr::TfToken("color"));
+        // Reset AOV to "color" on every delegate switch (mirrors stageView._handleRendererChanged)
+        m_currentAov = pxr::TfToken("color");
+        m_renderer->SetRendererAov(m_currentAov);
         LOG_INFO("Render delegate switched to: " + std::string(pluginId.GetText()));
     } else {
         LOG_ERROR("Failed to switch render delegate to: " + std::string(pluginId.GetText()));
@@ -351,6 +353,22 @@ bool UsdSceneRenderer::SetRendererPlugin(const pxr::TfToken& pluginId) {
     return ok;
 }
 
+pxr::TfTokenVector UsdSceneRenderer::GetRendererAovs() const
+{
+    if (!m_renderer) return {};
+    return m_renderer->GetRendererAovs();
+}
+
+bool UsdSceneRenderer::SetCurrentAov(const pxr::TfToken& aov)
+{
+    if (!m_renderer) return false;
+    if (m_renderer->SetRendererAov(aov)) {
+        m_currentAov = aov;
+        return true;
+    }
+    return false;
+}
+
 // ===========================================================================
 // Camera State
 // ===========================================================================

src/core/UsdSceneRenderer.h

@@ -183,6 +183,10 @@ public:
     bool              ShowCameraGuide()         const { return m_showCameraGuide; }
     void              SetShowCameraGuide(bool show)   { m_showCameraGuide = show; }
 
+    const pxr::TfToken& GetCurrentAov()        const { return m_currentAov; }
+    pxr::TfTokenVector  GetRendererAovs()       const;
+    bool                SetCurrentAov(const pxr::TfToken& aov);
+
     bool              GetAAEnabled()            const { return m_aaEnabled; }
     void              SetAAEnabled(bool enabled)      { m_aaEnabled = enabled; }
 
@@ -331,6 +335,7 @@ private:
 
     bool            m_showGrid;
     bool            m_showCameraGuide;
+    pxr::TfToken    m_currentAov;
     bool            m_aaEnabled;        ///< GL_LINE_SMOOTH anti-aliasing for all line overlays
     pxr::GfVec3f    m_backgroundColor;