Better MediaBox Support

This is an update of https://github.com/prawnpdf/prawn/pull/468.

It seems there are items in the ObjectStore that have an entry in Kids that does not have an entry in Parents. I'm not sure why this is, but it seems legitimate to search those nodes that call this node a Kid when searching for a MediaBox entry. This is a less aggressive approach as compared to the other pull request.

Again, I am happy to discuss whether or not this is the best solution for the problem at hand.