anchor.diff:  precision mediump float;
   #define LOWP lowp
   #else
  -#define LOWP 
  +#define LOWP
   #endif
   
   #ifdef TANGRAM_SDF_MULTISAMPLING
  -#extension GL_OES_standard_derivatives : enable
  +    #ifdef GL_OES_standard_derivatives
  +        #extension GL_OES_standard_derivatives : enable
  +    #else
  +        #undef TANGRAM_SDF_MULTISAMPLING
  +    #endif
   #endif
   
   uniform sampler2D u_tex;
  
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:     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);
  }
  
  