[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)

/* Copyright 2012 Mozilla Foundation
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http:www.apache.org/licenses/LICENSE-2.0