Disable jpeg chroma subsampling when saving with high quality

Currently it's not possible to generate a jpeg without chroma subsampling. It would be nice if Qt disabled chroma subsampling since some quality level.

To disable chroma subsampling with libjpeg:

cinfo.comp_info[0].v_samp_factor = 1;
cinfo.comp_info[0].h_samp_factor = 1;