[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)

c3 = tmp;
    }
    if (coords[p1 + 1] > coords[p2 + 1]) {
      tmp = p1;
      p1 = p2;
      p2 = tmp;
      tmp = c1;
      c1 = c2;
      c2 = tmp;
    }
    const x1 = (coords[p1] + context.offsetX) * context.scaleX;
    const y1 = (coords[p1 + 1] + context.offsetY) * context.scaleY;
    const x2 = (coords[p2] + context.offsetX) * context.scaleX;
    const y2 = (coords[p2 + 1] + context.offsetY) * context.scaleY;
    const x3 = (coords[p3] + context.offsetX) * context.scaleX;
    const y3 = (coords[p3 + 1] + context.offsetY) * context.scaleY;
    if (y1 >= y3) {
      return;
    }
    const c1r = colors[c1],
      c1g = colors[c1 + 1],
      c1b = colors[c1 + 2];
    const c2r = colors[c2],
      c2g = colors[c2 + 1],
      c2b = colors[c2 + 2];
    const c3r = colors[c3],
      c3g = colors[c3 + 1],
      c3b = colors[c3 + 2];
    const minY = Math.round(y1),
      maxY = Math.round(y3);