Otherwise terrain textures cache has zero hits because it stores not normalized
paths. Due to implicit conversion it's possible to add entry with
addEntryToObjectCache passing a string that is converted into normalized path.
But then getRefFromObjectCache called with original value does not find this
entry because it's not converted and overloaded operators are used instead.
NIFFile might not always be created from a file or stream containing NIF data.
Basically there are 2 different responsibilities for this class:
1. Read NIF file
2. Provide input for nifosg and bulletnifloader.
Remove no longer needed NIFFileMock since the state of NIFFfile can be
initialized independently from reading NIF file.
