Modify Luma / Chroma seperately

[technopagan]

From what I have seen apparently most of the work done is based on the characteristics of the Luma (Y) component and only the "global" quality setting is eventually decreased at no point the script tries to reduce Chroma quality independently of Luma (or even better Cr and Cb separately) since it has no way to measure if the result is qualitatively worse or not.

(submitted by Frédéric Kayser, f.kayser@free.fr)