This changes dtTileCache::update to add an optional parameter that
indicates whether the tile cache is up to date. This is useful to
determine whether calling the function again would do anything.
* Added virtual qualifier to a few destructors in subclasses.
* Removed a few empty destructors that didn't need to be explicitly defined.
* Fixed a few typos
* Removed unnecessary empty virtual destructors in some classes with parent's who derrived from a class with a virtual destructor
