[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)

if (!bbox || typeof Path2D === "undefined") {
    return;
  }
  const width = bbox[2] - bbox[0];
  const height = bbox[3] - bbox[1];
  const region = new Path2D();
  region.rect(bbox[0], bbox[1], width, height);
  ctx.clip(region);
}
ShadingIRs.RadialAxial = {
  fromIR: function RadialAxial_fromIR(raw) {
    const type = raw[1];
    const bbox = raw[2];
    const colorStops = raw[3];
    const p0 = raw[4];
    const p1 = raw[5];
    const r0 = raw[6];
    const r1 = raw[7];
    return {
      getPattern: function RadialAxial_getPattern(ctx) {
        applyBoundingBox(ctx, bbox);
        let grad;
        if (type === "axial") {
          grad = ctx.createLinearGradient(p0[0], p0[1], p1[0], p1[1]);
        } else if (type === "radial") {
          grad = ctx.createRadialGradient(p0[0], p0[1], r0, p1[0], p1[1], r1);
        }
        for (let i = 0, ii = colorStops.length; i < ii; ++i) {