assumptions about chroma subsampling should not be coded in as explicit
"divide by 4" (which assumes 4:2:0). Use the Y4M API to dynamically compute the chroma plane size. I.E. do not do 'w * h / 4' but instead use y4m_chroma_ss_x_ratio (and 'y').
Authored by: sms00 2005-05-26
Parent: [r2747]
Child: [r2749]
