fn composite(dib: &mut [u8], brain_alpha: u8)Expand description
Compose one frame into the DIB: solid BG fill, then the brain
blended on top at brain_alpha.
Brain bytes are pre-multiplied BGRA (alpha already baked into
RGB by build.rs). Modulating by brain_alpha / 255 keeps them
pre-multiplied for UpdateLayeredWindow’s AC_SRC_ALPHA blend.