niflib: Update to latest nif.xml file

include/gen/Polygon.h

include/obj/NiPSysMeshUpdateModifier.h

include/obj/NiPersistentSrcTextureRendererData.h

include/obj/NiPixelData.h

include/obj/NiScreenElements.h

include/obj/NiScreenElementsData.h

include/obj/NiSourceTexture.h

include/obj/bhkRigidBody.h

niflib.vcproj

niflib_VC2008.vcproj

src/gen/Polygon.cpp

src/gen/register.cpp

src/obj/NiPSysMeshUpdateModifier.cpp

src/obj/NiPersistentSrcTextureRendererData.cpp

src/obj/NiPixelData.cpp

src/obj/NiScreenElements.cpp

src/obj/NiScreenElementsData.cpp

src/obj/NiSourceTexture.cpp