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: 
  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));
  
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;
  