mp4: Provide whence parameter for the seek callback.

mp4: Provide whence parameter for the seek callback.

This adds a parameter to make ->seek() work like the lseek(2) system
call. This is easy to implement in both the memory-mapped callback
case used to retrieve the file information and the metadata update
case where ->seek() is a trivial wrapper for lseek(2).

With the additional functionality in place we don't need to track
the file size and the current file offset any more in mp4.c as these
values can now be obtained by calling ->seek() with a zero offset and
whence set to SEEK_END and SEEK_CUR, respectively. This also makes
the code more robust against corrupt mp4 files because we no longer
rely on the values from the atom headers to compute the file size.

The way mp4.c calls ->seek() should never cause the underlying lseek(2)
system call to fail. Therefore it suffices to check the return value
only in the callback wrapper and abort on failure.