Use more generic functions name that can work with more than one chroma. For
example, CopyFromYv12ToYv12 is renamed to Copy420_P_to_P (planar to planar).

Add picture_SwapUV(): just swap U, V planes of a tri-planar picture.

Remove CopyFromNv12ToYv12 (replaced by Copy420_SP_to_P() and picture_SwapUV()).

Add const qualifiers to all sources parameters in all functions.