Function encode_alpha 

pub fn encode_alpha(
    alpha: &[u8],
    width: u32,
    height: u32,
) -> CodecResult<Vec<u8>>

Encode alpha channel into an ALPH chunk payload.

alpha must contain exactly width * height bytes in row-major order. Returns the complete chunk payload: 1-byte header followed by the (optionally filtered) alpha data, using no compression.