Speed up QgsVirtualPointCloudEntity::handleSceneUpdate

Done by adding frustum culling early in the loop, results in a ~10x
speedup in one project.

src/3d/qgsvirtualpointcloudentity_p.cpp

@@ -130,9 +130,18 @@ void QgsVirtualPointCloudEntity::handleSceneUpdate( const SceneContext &sceneCon
   const QVector<QgsPointCloudSubIndex> subIndexes = provider()->subIndexes();
   for ( int i = 0; i < subIndexes.size(); ++i )
   {
+    // If the chunked entity needs an update, do it even if it's occluded,
+    // since otherwise we'd return needsUpdate() == true until it comes into
+    // view again.
+    bool needsUpdate = mChunkedEntitiesMap.contains( i ) && mChunkedEntitiesMap[i]->needsUpdate();
+
     const QgsBox3D &box3D = mBboxes.at( i );
 
-    if ( box3D.isEmpty() )
+    if ( !needsUpdate && box3D.isEmpty() )
+      continue;
+
+    QgsAABB aabb = QgsAABB::fromBox3D( box3D, mBboxesEntity->vertexDataOrigin() );
+    if ( !needsUpdate && Qgs3DUtils::isCullable( aabb, sceneContext.viewProjectionMatrix ) )
       continue;
 
     // magic number 256 is the common span value for a COPC root node