[lines_issue.pdf](https://github.com/mozilla/pdf.js/files/3441553/lines_issue.pdf)

Configuration:
- Chrome (latest)
- Mac Os X Mojac
- PDF.js version: latest (reproduced on pdf.js demo page)

Steps to reproduce the problem:
1. Open attached pdf in pdf.js demo viewer
2. Please note that thin line in the middle is not affected by gradient, but it should be (see screenshot below taken from Mac Os X Preview)

What is the expected behavior? (add screenshot)

![image](https://user-images.githubusercontent.com/430887/62041871-5d375a00-b205-11e9-9811-97c1643ae5b9.png)

What went wrong? (add screenshot)

![image](https://user-images.githubusercontent.com/430887/62041851-51e42e80-b205-11e9-97a3-5a1ef1b20245.png)

);
    let splitXBy = Math.ceil(
      ((figureMaxX - figureMinX) * TRIANGLE_DENSITY) /
        (mesh.bounds[2] - mesh.bounds[0])
    );
    splitXBy = Math.max(
      MIN_SPLIT_PATCH_CHUNKS_AMOUNT,
      Math.min(MAX_SPLIT_PATCH_CHUNKS_AMOUNT, splitXBy)
    );
    let splitYBy = Math.ceil(
      ((figureMaxY - figureMinY) * TRIANGLE_DENSITY) /
        (mesh.bounds[3] - mesh.bounds[1])
    );
    splitYBy = Math.max(
      MIN_SPLIT_PATCH_CHUNKS_AMOUNT,
      Math.min(MAX_SPLIT_PATCH_CHUNKS_AMOUNT, splitYBy)
    );
    const verticesPerRow = splitXBy + 1;
    const figureCoords = new Int32Array((splitYBy + 1) * verticesPerRow);
    const figureColors = new Int32Array((splitYBy + 1) * verticesPerRow);
    let k = 0;
    const cl = new Uint8Array(3),
      cr = new Uint8Array(3);
    const c0 = colors[ci[0]],
      c1 = colors[ci[1]],
      c2 = colors[ci[2]],
      c3 = colors[ci[3]];
    const bRow = getB(splitYBy),
      bCol = getB(splitXBy);
    for (let row = 0; row <= splitYBy; row++) {