mediump float blend_stroke(mediump float base, mediump float stroke, int mode)
{
   if      (mode == 0) /* normal      */ return (base + stroke) * 0.5;
   else if (mode == 1) /* multiply    */ return base * stroke;
   else if (mode == 2) /* screen      */ return 1.0-(1.0-base)*(1.0-stroke);
   else if (mode == 3) /* color-dodge */ return base/(1.0-stroke);
   else if (mode == 4) /* overlay     */ return mix(2.0*base*stroke, 1.0-2.0*(1.0-base)*(1.0-stroke), floor(base*2.0));
   else return 1.0;
}