TComPic: clean up data member ordering, add comments
weightp rewrite without a class

weightp analysis for the main encoder is performed with motion compensation
(using lowres motion vectors from lookahead) when available.  The lowres
luma plane is used for luma analysis and the chroma planes are analysed as-is.
weightp: extend chroma borders before mcChroma, remove redundant checks
TComPic: add m_chromaPlanesExtended member var
weightp: avoid redundant chroma extensions