函数 av_chroma_location_enum_to_pos 

pub unsafe extern "C" fn av_chroma_location_enum_to_pos(
    xpos: *mut c_int,
    ypos: *mut c_int,
    pos: AVChromaLocation,
) -> c_int

Converts AVChromaLocation to swscale x/y chroma position.

The positions represent the chroma (0,0) position in a coordinates system with luma (0,0) representing the origin and luma(1,1) representing 256,256

@param xpos horizontal chroma sample position @param ypos vertical chroma sample position