anchor.diff:      if (v_alpha == 0.0) {
           discard;
       }
  -    
  +
       float distance = texture2D(u_tex, v_uv).a;
       float alpha = sampleAlpha(v_uv, distance, sdf) * tint;
       alpha = pow(alpha, 1.0 / gamma);
  -    
  +
       gl_FragColor = vec4(u_color, v_alpha * alpha);
   }
  +
  
positive.code:                    + sample(box.zw, distance, off)
                     + sample(box.xw, distance, off)
                     + sample(box.zy, distance, off);
          
          alpha = (alpha + 0.5 * asum) / 2.0;
      #endif
  
  
negative.code: precision mediump float;
  #define LOWP lowp
  #else
  #define LOWP
  #endif
  
  #ifdef TANGRAM_SDF_MULTISAMPLING
      #ifdef GL_OES_standard_derivatives
          #extension GL_OES_standard_derivatives : enable
      #else
          #undef TANGRAM_SDF_MULTISAMPLING
      #endif
  #endif
  
  uniform sampler2D u_tex;
  