Function encode_image_gray_alpha_16bit 

pub fn encode_image_gray_alpha_16bit(
    input: &[u16],
    width: usize,
    height: usize,
    config: &EncodeConfig,
) -> Result<Vec<u8>, EncodeError>

Encode a 16-bit grayscale+alpha image. input is interleaved [L, A] pairs, width * height * 2 samples total, each in 0..=65535.