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
const paths = this.pendingTextPaths || (this.pendingTextPaths = []);
        paths.push({
          transform: ctx.mozCurrentTransform,
          x,
          y,
          fontSize,
          addToPath,
        });
      }
    },
    get isFontSubpixelAAEnabled() {
      
      
      const { context: ctx } = this.cachedCanvases.getCanvas(
        "isFontSubpixelAAEnabled",
        10,
        10
      );
      ctx.scale(1.5, 1);
      ctx.fillText("I", 0, 10);
      const data = ctx.getImageData(0, 0, 10, 10).data;
      let enabled = false;
      for (let i = 3; i < data.length; i += 4) {
        if (data[i] > 0 && data[i] < 255) {
          enabled = true;
          break;
        }
      }
      return shadow(this, "isFontSubpixelAAEnabled", enabled);
    },