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
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      if (size < 0) {
        size = -size;
        current.fontDirection = -1;
      } else {
        current.fontDirection = 1;
      }
      this.current.font = fontObj;
      this.current.fontSize = size;
      if (fontObj.isType3Font) {
        return; 
      }
      const name = fontObj.loadedName || "sans-serif";
      let bold = "normal";
      if (fontObj.black) {
        bold = "900";
      } else if (fontObj.bold) {
        bold = "bold";
      }
      const italic = fontObj.italic ? "italic" : "normal";
      const typeface = `"${name}", ${fontObj.fallbackName}`;
      
      
      
      
      let browserFontSize = size;
      if (size < MIN_FONT_SIZE) {
        browserFontSize = MIN_FONT_SIZE;