static Sk4f premul(const Sk4f& x) { return x * scale_rgb(x.kth<SkPM4f::A>()); }
static Sk4f unpremul(const Sk4f& x) { return x * scale_rgb(1 / x.kth<SkPM4f::A>()); // TODO: fast/approx invert? }
static Sk4f premul(const Sk4f& x) { return x * scale_rgb(x[SkPM4f::A]); }