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
? fillColor.getPattern(maskCtx, this)
        : fillColor;
      maskCtx.fillRect(0, 0, width, height);
      maskCtx.restore();
      const ctx = this.ctx;
      for (let i = 0, ii = positions.length; i < ii; i += 2) {
        ctx.save();
        ctx.transform(
          scaleX,
          skewX,
          skewY,
          scaleY,
          positions[i],
          positions[i + 1]
        );
        ctx.scale(1, -1);
        ctx.drawImage(maskCanvas.canvas, 0, 0, width, height, 0, -1, 1, 1);
        ctx.restore();
      }
    },
    paintImageMaskXObjectGroup: function CanvasGraphics_paintImageMaskXObjectGroup(
      images
    ) {
      if (!this.contentVisible) {
        return;
      }
      const ctx = this.ctx;
      const fillColor = this.current.fillColor;
      const isPatternFill = this.current.patternFill;
      for (let i = 0, ii = images.length; i < ii; i++) {