mp4: Kill membuffer API.

mp4: Kill membuffer API.

Thanks to the previous cleanups, create_ilst() is the last remaining
membuffer user. Since the size of the ilst atom can be computed as the
sum of the tag lengths plus a constant times the number of tag items,
we can allocate a suitably sized buffer up-front instead of relying
on the membuffer framework to allocate and resize buffers as needed.