Forgot to mention it in a previous commit, but I added some template functions for extracting flags and fields from bitfields to NIF_IO.h.  I just put them through some tests and they appear to work correctly.

Finished new MatTexCollection helper class which makes it possible to manipulate material properties and texture objects with the same classes regardless of the intended NIF version.
Realized that only read was implemented for files with version < 3.3.0.13, so implemented write for those old versions as well.