Function encode_rgba8_opaque 

pub fn encode_rgba8_opaque(
    width: u32,
    height: u32,
    rgba: &[u8],
    quality: u8,
) -> Result<Vec<u8>, ImageError>

Encode rgba (width * height * 4 bytes) as a JPEG of quality, dropping its alpha channel: JPEG keeps none. Each pixel keeps its colour values as they are, uncomposited.