[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)

const [x1, y1, r1, x2, y2, r2] = this.coordsArr;
      const distance = Math.hypot(x1 - x2, y1 - y2);
      if (r1 <= r2 + distance && r2 <= r1 + distance) {
        warn("Unsupported radial gradient.");
      }
    }
    this.extendStart = extendStart;
    this.extendEnd = extendEnd;
    const fnObj = dict.getRaw("Function");
    const fn = pdfFunctionFactory.createFromArray(fnObj);
    
    
    
    const NUMBER_OF_SAMPLES = 10;
    const step = (t1 - t0) / NUMBER_OF_SAMPLES;
    const colorStops = (this.colorStops = []);
    
    if (t0 >= t1 || step <= 0) {
      
      
      info("Bad shading domain.");
      return;
    }
    const color = new Float32Array(cs.numComps),
      ratio = new Float32Array(1);
    let rgbColor;
    for (let i = 0; i <= NUMBER_OF_SAMPLES; i++) {
      ratio[0] = t0 + i * step;