cuvid: Add MIT licenced nvcuid headers from Video SDK 7.0

For unknown reasons, the only accurately descriptive version of
cuviddec.h is in the Video SDK - the one in CUDA 7.5 lacks vp8
PICPARAMS and the vp9 struct definition is inaccurate. The CUDA 8 RC
includes an ancient version of this file from many many years go.

However, the one in the Video SDK is modified to work through a
dynamic link mechanism which we don't really want to use, so the
next change will modify the files to just declare functions in
the normal way.

I've split the changes so it's clear to see what changed between
the original files and ones that work for us.