Add y4m_get_chroma_ss() function. Returns the subsampling factors (i.e. for
411 those are 4 and 1, for 422 those are 2 and 2, and so on). Useful for programs that need to make sure counts are a proper multiple of the chroma subsampling factor.
Authored by: sms00 2004-04-04
Parent: [r2280]
Child: [r2282]
