Steps to reproduce the problem:
1. When using the demo application - https://mozilla.github.io/pdf.js/web/viewer.html, load  the document 5311693-1.pdf

[5311693-1.pdf](https://github.com/mozilla/pdf.js/files/5137225/5311693-1.pdf)

2. go to Page 16 of the document.

3.  The strip of  EKG data  appears to have an incorrect line width   under the  Multi-color heat rate chart.

4.  When the same document is generated by pdffactory ,  the artifact are not visible.

What is the expected behavior? (add screenshot)
![expected](https://user-images.githubusercontent.com/3577617/91461157-4622cb80-e84e-11ea-8e5d-750c7b4e64ae.PNG)

What went wrong? (add screenshot)
The EKG line strip  has an incorrect thickness.  the x-transform seems to stretch the line thickness.  See the  stri in the RED rectagngle of the attached behavior versus the GREEN rectangle

Link to a viewer (if hosted on a site other than mozilla.github.io/pdf.js or as Firefox/Chrome extension):
 https://mozilla.github.io/pdf.js/web/viewer.html
m[0] * c + m[2] * d,
        m[1] * c + m[3] * d,
        m[0] * e + m[2] * f + m[4],
        m[1] * e + m[3] * f + m[5],
      ];
      ctx._originalTransform(a, b, c, d, e, f);
    };
    ctx.setTransform = function ctxSetTransform(a, b, c, d, e, f) {
      this._transformMatrix = [a, b, c, d, e, f];
      ctx._originalSetTransform(a, b, c, d, e, f);
    };
    ctx.rotate = function ctxRotate(angle) {
      const cosValue = Math.cos(angle);
      const sinValue = Math.sin(angle);
      const m = this._transformMatrix;
      this._transformMatrix = [
        m[0] * cosValue + m[2] * sinValue,
        m[1] * cosValue + m[3] * sinValue,
        m[0] * -sinValue + m[2] * cosValue,
        m[1] * -sinValue + m[3] * cosValue,
        m[4],
        m[5],
      ];
      this._originalRotate(angle);
    };
  }
}
const CachedCanvases = (function CachedCanvasesClosure() {
  
  function CachedCanvases(canvasFactory) {