SceneCacheData: Add support for root location tags added to the internal root.

In USD, we can't add collections to the absolute root `/` so we leverage
the internal root (used to work around the limitation with referencing
layer in USD that get read of the file's pseudo root) to store the
tags/collections.

Author: lucienfostier

contrib/IECoreUSD/src/IECoreUSD/SceneCacheData.cpp

@@ -217,7 +217,7 @@ void SceneCacheData::addReference( ConstSceneInterfacePtr scene, SpecData& spec,
 	addValueClip( spec, times, actives, linkFileName, linkRootPath.GetText() );
 }
 
-void SceneCacheData::addInternalRoot( TfTokenVector children )
+void SceneCacheData::addInternalRoot( TfTokenVector children, IECoreScene::ConstSceneInterfacePtr scene )
 {
 	// add transform for internal root.
 	SdfPath internalRootPath = SdfPath::AbsoluteRootPath().AppendChild( SceneCacheDataAlgo::internalRootNameToken() );
@@ -237,6 +237,26 @@ void SceneCacheData::addInternalRoot( TfTokenVector children )
 	// steal root children
 	internalRootSpec.fields.push_back( FieldValuePair( SdfChildrenKeys->PrimChildren, children ) );
 
+	// add collection
+	FieldValuePair propertyChildren;
+	propertyChildren.first = SdfChildrenKeys->PropertyChildren;
+	TfTokenVector properties;
+
+	// we don't want to keep children tags in this case.
+	m_collections.clear();
+
+	SceneInterface::NameList tags;
+	scene->readTags( tags );
+	for ( auto& tag : tags )
+	{
+		m_collections[tag].push_back( internalRootPath );
+	}
+
+	addCollections( internalRootSpec, properties, internalRootPath );
+
+	propertyChildren.second = properties;
+	internalRootSpec.fields.push_back( propertyChildren );
+
 	m_data[internalRootPath] = internalRootSpec;
 }
 
@@ -344,7 +364,7 @@ void SceneCacheData::loadSceneIntoCache( ConstSceneInterfacePtr scene )
 		// end timecode
 		spec.fields.push_back( FieldValuePair( SdfFieldKeys->EndTimeCode, lastFrame ) );
 
-		addInternalRoot( children );
+		addInternalRoot( children, scene );
 
 		// add internal root as single child
 		children.clear();

contrib/IECoreUSD/src/IECoreUSD/SceneCacheData.h

@@ -179,7 +179,7 @@ class SceneCacheData : public SdfAbstractData
 	void addCollections( SpecData& spec, TfTokenVector& properties, const SdfPath& primPath );
 	void addReference( IECoreScene::ConstSceneInterfacePtr scene, SpecData& spec, TfTokenVector& children );
 	void addValueClip( SpecData& spec, const VtVec2dArray times, const VtVec2dArray actives, const std::string& assetPath, const std::string& primPath);
-	void addInternalRoot( TfTokenVector children );
+	void addInternalRoot( TfTokenVector children, IECoreScene::ConstSceneInterfacePtr scene );
 
 	VtValue getTimeSampleMap( const SdfPath& path, const TfToken& field, const VtValue& value ) const;