Renders the scene graph, described by a tree of `GskRenderNode` instances to the renderer's surface, ensuring that the given region
gets redrawn.
If the renderer has no associated surface, this function does nothing.
Renderers must ensure that changes of the contents given by the root node as well as the area given by region
are redrawn. They are however free to not redraw any pixel outside of region if they can guarantee that it didn't change.
The this will acquire a reference on the `GskRenderNode` tree while the rendering is in progress.
| this |
a realized `GskRenderer` |
| root |
a `GskRenderNode` |
| region |
the `cairo_region_t` that must be redrawn or null for the whole window |