Retrieves a `GdkCursor` pointer for the cursor currently set on the `GdkSurface`.
If the return value is null then there is no custom cursor set on the surface, and it is using the cursor for its parent surface.
a `GdkCursor`, or null. The returned object is owned by the `GdkSurface` and should not be unreferenced directly. Use [method@Gdk.Surface.set_cursor] to unset the cursor of the surface