Function downsample_2x 

pub fn downsample_2x(
    input: &str,
    output: &str,
    width: u32,
    height: u32,
) -> Program

Build a Program that 2× downsamples input into output.

• input: [u32; width * height] - source pixels (packed RGBA)
• output: [u32; (width/2) * (height/2)] - downsampled result
• Width and height must be even.