Snapshot the a child of this.
When a widget receives a call to the snapshot function, it must send synthetic `GtkWidget`Class.snapshot() calls to all children. This function
provides a convenient way of doing this. A widget, when it receives a call to its `GtkWidget`Class.snapshot() function, calls
snapshot_child once for each child, passing in the
snapshot the widget received.
snapshot_child takes care of translating the origin of
snapshot, and deciding whether the child needs
to be snapshot.
This function does nothing for children that implement `GtkNative`.
a child of this