Function compose_src_over 

fn compose_src_over(src: PremulRgba8, dst: PremulRgba8) -> PremulRgba8

Composite two pixels using Porter-Duff “source over” operator.

Composes the source pixel over the destination pixel using premultiplied alpha blending. Returns the composited result.

Formula for premultiplied colors: result = src + dst * (1 - src_alpha)