isom: convert mp4 dvdsub extradata to vobsub format

mp4 files embedding DVD subtitles do not use the same extradata format
as the rest of Libav expects. The subtitle decoder in libavcodec in
particular does not understand this format.

Convert the extradata to the vobsub .idx format. mp4 stores the palette
as binary 32 bit ints in YUV. The subtitle resolution is stored
separately in the track header, which we access through AVStream.

Signed-off-by: Anton Khirnov <anton@khirnov.net>