If I have a graph like:
  src -> ... -> crop (x1, y1, w1, h1) -> ... -> ouput

Then changing the properties of the crop to (x2, y2, w2, h2) leads to GeglNode::invalidated being emitted four times with some misleading rectangles. This is because gegl_node_property_changed takes the bounding box of the rectangles before and after the change. In this case it leads to:
  (i) bbox ((x1, y1, w1, h1), (x2, y1, w1, h1))
 (ii) bbox ((x1, y1, w1, h1), (x2, y2, w1, h1))
(iii) bbox ((x1, y1, w1, h1), (x2, y2, w2, h1))
 (iv) bbox ((x1, y1, w1, h1), (x2, y2, w2, h2))

All but (iv) are bogus, and can be confusing when debugging. We should only emit invalidated with the rectangle in (iv).

Freezing the GeglOperation during property changes is one way to achieve that.