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
this.canvasFactory = canvasFactory;
    this.cache = Object.create(null);
  }
  CachedCanvases.prototype = {
    getCanvas: function CachedCanvases_getCanvas(
      id,
      width,
      height,
      trackTransform
    ) {
      let canvasEntry;
      if (this.cache[id] !== undefined) {
        canvasEntry = this.cache[id];
        this.canvasFactory.reset(canvasEntry, width, height);
        
        canvasEntry.context.setTransform(1, 0, 0, 1, 0, 0);
      } else {
        canvasEntry = this.canvasFactory.create(width, height);
        this.cache[id] = canvasEntry;
      }
      if (trackTransform) {
        addContextCurrentTransform(canvasEntry.context);
      }
      return canvasEntry;
    },
    clear() {