gegl:wind: Port to gegl_random, other cleanup

* Use iterators instead of linear_open for input calculations. Which
is not the most efficient thing, but avoids linearizing the entire
input buffer.
* Use gegl_malloc instead of g_slice.
Add gegl_node_get_gegl_operation

While obscured behind a property the operation object of GeglNode
was already public, this just provides a more convenient access
point for it.