hd: emulate non-hierarchical removal in render index

Originally, when HdSceneDelegate removed a prim from the render index
via RemoveRprim, RemoveSprim or RemoveBprim, it did not remove any
children.  HdRenderIndex::RemoveSprim was updated in a previous commit
to reproduce the flat behavior but other prim types were left
unchanged.  This change moves the RemoveSprim behavior into
HdLegacyPrimSceneIndex so that removing Sprims, Rprims and Bprims are
handled uniformly in scene index emulation.

(Internal change: 2262307)

pxr/imaging/hd/legacyPrimSceneIndex.cpp

@@ -35,6 +35,17 @@ HdLegacyPrimSceneIndex::AddLegacyPrim(SdfPath const &id, TfToken const &type,
         HdDataSourceLegacyPrim::New(id, type, sceneDelegate)}});
 }
 
+void
+HdLegacyPrimSceneIndex::RemovePrim(SdfPath const &id)
+{
+    if (!GetChildPrimPaths(id).empty()) {
+        AddPrims({{id, TfToken(), nullptr}});
+    }
+    else {
+        RemovePrims({id});
+    }
+}
+
 void
 HdLegacyPrimSceneIndex::DirtyPrims(
     const HdSceneIndexObserver::DirtiedPrimEntries &entries)

pxr/imaging/hd/legacyPrimSceneIndex.h

@@ -53,6 +53,18 @@ class HdLegacyPrimSceneIndex : public HdRetainedSceneIndex
     void AddLegacyPrim(SdfPath const &id, TfToken const &type,
         HdSceneDelegate *sceneDelegate);
 
+    /// Remove only the prim at \p id without affecting children.
+    ///
+    /// If \p id has children, it is replaced by an entry with no type
+    /// and no data source.  If \p id does not have children, it is
+    /// removed from the retained scene index.
+    ///
+    /// This is called by HdRenderIndex on behalf of legacy
+    /// HdSceneDelegates to emulate the original behavior of
+    /// Remove{B,R,S}Prim, which did not remove children.
+    ///
+    void RemovePrim(SdfPath const &id);
+
     /// extends to also call DirtyPrim on HdDataSourceLegacyPrim
     void DirtyPrims(
         const HdSceneIndexObserver::DirtiedPrimEntries &entries) override;

pxr/imaging/hd/renderIndex.cpp

@@ -352,7 +352,7 @@ HdRenderIndex::RemoveRprim(SdfPath const& id)
     // If we are emulating let's remove from the scene index
     // which will trigger render index removals later.
     if (_IsEnabledSceneIndexEmulation()) {
-        _emulationSceneIndex->RemovePrims({id});
+        _emulationSceneIndex->RemovePrim(id);
         return;
     }
 
@@ -643,18 +643,7 @@ void
 HdRenderIndex::RemoveSprim(TfToken const& typeId, SdfPath const& id)
 {
     if (_IsEnabledSceneIndexEmulation()) {
-
-        // Removing an sprim doesn't remove any descendant prims from the
-        // renderIndex. Removing a prim from the scene index does remove
-        // all descendant prims. Special case removal of an sprim which has
-        // children to instead be replaced with an empty type.
-        if (!_emulationSceneIndex->GetChildPrimPaths(id).empty()) {
-             _emulationSceneIndex->AddPrims({{id, TfToken(), nullptr}});
-             return;
-        }
-
-        _emulationSceneIndex->RemovePrims({id});
-
+        _emulationSceneIndex->RemovePrim(id);
         return;
     }
 
@@ -727,7 +716,7 @@ void
 HdRenderIndex::RemoveBprim(TfToken const& typeId, SdfPath const& id)
 {
     if (_IsEnabledSceneIndexEmulation()) {
-        _emulationSceneIndex->RemovePrims({id});
+        _emulationSceneIndex->RemovePrim(id);
         return;
     }