anchor.diff:  
   float sampleAlpha(in vec2 uv, float distance, in float off) {
       float alpha = contour(distance, distance, off);
  -    
  +
       #ifdef TANGRAM_SDF_MULTISAMPLING
           float dscale = 0.354; // 1 / sqrt(2)
           vec2 duv = dscale * (dFdx(uv) + dFdy(uv));
  
positive.code: precision mediump float;
  #define LOWP lowp
  #else
  #define LOWP 
  #endif
  
  #ifdef TANGRAM_SDF_MULTISAMPLING
  #extension GL_OES_standard_derivatives : enable
  #endif
  
  uniform sampler2D u_tex;
  
negative.code:                    + sample(box.zw, distance, off)
                     + sample(box.xw, distance, off)
                     + sample(box.zy, distance, off);
  
          alpha = (alpha + 0.5 * asum) / 2.0;
      #endif
  
  