pre_mul_blend_channel
out[i].r = pre_mul_blend_channel(in[i].r, out[i].r, in[i].a);
out[i].g = pre_mul_blend_channel(in[i].g, out[i].g, in[i].a);
out[i].b = pre_mul_blend_channel(in[i].b, out[i].b, in[i].a);