movenc: Don't add 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 libavformat
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: Martin Storsjö <martin@martin.st>
