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 measuredWidth =
            ((ctx.measureText(character).width * 1000) / fontSize) *
            fontSizeScale;
          if (width < measuredWidth && this.isFontSubpixelAAEnabled) {
            const characterScaleX = width / measuredWidth;
            restoreNeeded = true;
            ctx.save();
            ctx.scale(characterScaleX, 1);
            scaledX /= characterScaleX;
          } else if (width !== measuredWidth) {
            scaledX +=
              (((width - measuredWidth) / 2000) * fontSize) / fontSizeScale;
          }
        }
        
        
        if (this.contentVisible && (glyph.isInFont || font.missingFile)) {
          if (simpleFillText && !accent) {
            
            ctx.fillText(character, scaledX, scaledY);
          } else {
            this.paintChar(character, scaledX, scaledY, patternTransform);
            if (accent) {
              const scaledAccentX =
                scaledX + (fontSize * accent.offset.x) / fontSizeScale;
              const scaledAccentY =
                scaledY - (fontSize * accent.offset.y) / fontSizeScale;