Scene root hash fixes

A problem was recently discovered where if an attributes plug was pulled on at the scene root, all subsequent pulls would incorrectly return an empty compound object. Transform and object plugs suffered from similar problems.

This was because SceneNode::compute was special cased for the scene root, but these special cases were not reflected in SceneNode::hash, which was confusing the caching system. This is addressed by hashing the default plug values for the special cased plugs at the root.