* initial positioning in a subsection of a file failed. This mainly affected music playback.
* made the FileRdr constructor which takes a FileReaderInterface private so that everything that needs it must be explicitly declared as friend.
* removed a few redundant construction initializers for FileRdrs.
* loading compressed nodes needs to check the validity of its reader.
* use GetLength to detemine the size of a Zip file instead of doing another seek to the end.
* removed duplicate Length variables.
