renderTexture 
  open fun renderTexture(root: RenderNode, viewport: <Error class: unknown class>? = null): <Error class: unknown class>(source)
Renders the scene graph, described by a tree of GskRenderNode instances, to a GdkTexture.
The @renderer will acquire a reference on the GskRenderNode tree while the rendering is in progress.
If you want to apply any transformations to @root, you should put it into a transform node and pass that node instead.
Return
a GdkTexture with the rendered contents of @root.
Parameters
root
a GskRenderNode
viewport
the section to draw or null to use @root's bounds