- fixed some remaining issues:

* 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.

src/m_png.cpp

@@ -75,7 +75,7 @@ struct IHDR
 	uint8_t		Interlace;
 };
 
-PNGHandle::PNGHandle (FileRdr &file) : File(0), bDeleteFilePtr(true), ChunkPt(0)
+PNGHandle::PNGHandle (FileRdr &file) : bDeleteFilePtr(true), ChunkPt(0)
 {
 	File = std::move(file);
 }