- a bit of header cleanup.
* moved <zlib.h> and <bzlib.h> from files.h to files_decompress.cpp because they are no longer needed for defining the interface.
* added <functional> to the precompiled header
* 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.
