Renders the scene graph, described by a tree of `GskRenderNode` instances, to a `GdkTexture`.
The this 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.
| this |
a realized `GskRenderer` |
| root |
a `GskRenderNode` |
| viewport |
the section to draw or null to use |
|
a `GdkTexture` with the rendered contents of |