m_textureProviderConnection = connect(provider, &QSGTextureProvider::textureChanged, this, [provider, imageNode] () {
                    // called on the render thread, if there is one; while not
                    // obvious, the gui thread is blocked too because one can
                    // get here only from either the textureProvider() call
                    // above, or from QQuickImage::updatePaintNode()
                    imageNode->m_qsgTexture = provider->texture();
                    // the QSGTexture may be different now, go through loadRenderImage() again
                    imageNode->m_flags.setFlag(QSSGRenderImage::Flag::Dirty);
                }, Qt::DirectConnection);

    View3D {
         id: lightmapSource
         renderMode: View3D.Offscreen // so a texture provider as well
         ...
    }
    ...
    View3D {
        ...
        Model {
            ...
            materials: DefaultMaterial {
                lighting: DefaultMaterial.NoLighting
                diffuseMap: Texture {
                    sourceItem: lightmapSource
                    indexUV: 1
                }
            }
        }
    }