Changed the way we determine should an object be meshed or not.

Changed the way we determine should an object be meshed or not.
* By the time we hit set in BasePrim value and BasePrim are the same object
  (libomv doesn't create a new object on update if it already knows about it)
* Back to keep track of hash codes, this time only on object update so
  the impact is negligible

git-svn-id: https://radegast.googlecode.com/svn/trunk@1046 f7a694da-4d33-11de-9ad6-1127a62b9fcd