message: Use center of the pixel when aligning undercurl
  Since `x` position in rect shader represents left side of the pixel we
  should use the center of it when dealing with contiguous functions.
diff: @@ -37,11 +37,14 @@ uniform float_t undercurlPosition;
  color_t draw_undercurl(float_t x, float_t y) {
  // We use `undercurlPosition` as an amplitude, since it's half of the descent
  // value.
  - float_t undercurl = undercurlPosition / 2. * cos(x * 2. * PI / cellWidth)
  - + undercurlPosition - 1.;
  -
  - float_t undercurlTop = undercurl + max((underlineThickness - 1.), 0.);
  - float_t undercurlBottom = undercurl - max((underlineThickness - 1.), 0.);
  + //
  + // The `x` represents the left bound of pixel we should add `1/2` to it, so
  + // we compute the undercurl position for the center of the pixel.
  + float_t undercurl = undercurlPosition / 2. * cos((x + 0.5) * 2.
  + * PI / cellWidth) + undercurlPosition - 1.;
  +
  + float_t undercurlTop = undercurl + max((underlineThickness - 1.), 0.) / 2.;
  + float_t undercurlBottom = undercurl - max((underlineThickness - 1.), 0.) / 2.;
  // Compute resulted alpha based on distance from `gl_FragCoord.y` to the
  // cosine curve.
  