[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 currentTransform = currentCtx.mozCurrentTransform;
      if (group.matrix) {
        currentCtx.transform.apply(currentCtx, group.matrix);
      }
      if (!group.bbox) {
        throw new Error("Bounding box is required.");
      }
      
      
      let bounds = Util.getAxialAlignedBoundingBox(
        group.bbox,
        currentCtx.mozCurrentTransform
      );
      
      const canvasBounds = [
        0,
        0,
        currentCtx.canvas.width,
        currentCtx.canvas.height,
      ];
      bounds = Util.intersect(bounds, canvasBounds) || [0, 0, 0, 0];
      
      
      const offsetX = Math.floor(bounds[0]);
      const offsetY = Math.floor(bounds[1]);
      let drawnWidth = Math.max(Math.ceil(bounds[2]) - offsetX, 1);
      let drawnHeight = Math.max(Math.ceil(bounds[3]) - offsetY, 1);
      let scaleX = 1,
        scaleY = 1;
      if (drawnWidth > MAX_GROUP_SIZE) {