anchor.diff:  uniform float u_time;
   uniform float u_meters_per_pixel;
   uniform float u_device_pixel_ratio;
  +uniform float u_max_stroke_width;
   uniform int u_pass;
  -uniform float u_max_stroke_width;
   
   #pragma tangram: uniforms
   
  
positive.code: #pragma tangram: extensions
  
  #ifdef TANGRAM_SDF_MULTISAMPLING
      #ifdef GL_OES_standard_derivatives
          #extension GL_OES_standard_derivatives : enable
      #else
          #undef TANGRAM_SDF_MULTISAMPLING
      #endif
  #endif
  
  #ifdef GL_ES
      precision mediump float;
  
negative.code:     // - sdf_scale == fontScale / glyphScale:
      //   When the glyph is scaled down, 's' must be increased
      //   (used to interpolate 1px of the scaled glyph around v_sdf_threshold)
  
      float sdf_pixel = 0.5 / (u_max_stroke_width * v_sdf_scale);
      float add_smooth = 0.25;
      float filter_width = (sdf_pixel * (0.5 + add_smooth));
  
      float start = max(v_sdf_threshold - filter_width, 0.0);
      float end = v_sdf_threshold + filter_width;
  
      float alpha = smoothstep(start, end, signed_distance);
  
      color.a *= v_alpha * alpha;
  
      #pragma tangram: color
      #pragma tangram: filter
  