Make it possible to set parameters in jpeg_compress_struct from user applications

When it writes a JPEG file, QJpegHandler internally and automatically sets most of parameters in jpeg_compress_struct.
Thus, for example, parameters such as "write_JFIF_header" or "comp_info[0].v_samp_factor" are not settable directly from user applications. write_JFIF_header is set as 'TRUE' and comp_info[0].v_samp_factor is set as '2' inside of  QJpegHandler::write().
This prevent some users to achieve their preferable settings.

It would be good if there is a way for user applications to set those parameters by themselves.