Provide our own copy of libmp4ff.

Provide our own copy of libmp4ff.

This library was removed from the faad repository in 2017. This
commit put everything we need from the libmp4ff code base into the
new mp4.c and mp4.h files. All content was taken unmodified from
commit 1cf5a505daf0 of the faad2 repo, the last commit that contains
the library.

The code needs a lot of cleanup and does not compile without warnings.
We commit it as-is to track what exactly has been copied from
libmp4ff. The new mp4.c has not been included into the build system,
however, so this commit has no effect yet.