chroma: copy: rename YUV420 copy functions
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.
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