Turn off usage of base_data_offset in tfhd for PIFF files

According to the PIFF specification[1] the base_data_offset field MUST be
omitteed. See section 5.2.17. Since the ISMV files created by ffmpeg state
that they are 'piff' compatible via 'ftyp' box, this needs to be corrected.

[1] http://www.iis.net/learn/media/smooth-streaming/protected-interoperable-file-format

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>