Comment #9 on issue 576 by tudor.gi...(a)gmail.com: Border of children
depends on another shape in a complex shape
http://code.google.com/p/moose-technology/issues/detail?id=576
Here is an idea. In VW, we had a cache only for bounds. That can be a one
level dictionary with a shape as a key. Then you can use the two level
cache for the other properties that do not necessarily affect the bounds.