Need to seek to start of jpeg2000 data in a stream

[tclarke]

Issue #198 removed the opj_stream_create_file_stream() variant which takes a FILE*. I have been using this to fseek() to the location within a file where the jpeg2000 data resides. There does not appear to be a way to perform a seek on the file. I see functions (opj_stream_read_seek and opj_stream_write_seek) to do this but they are not exposed in the public API. There should be some way of seeking on an open opj stream.