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
ctx.save();
        ctx.transform.apply(ctx, entry.transform);
        ctx.scale(1, -1);
        ctx.drawImage(
          tmpCanvas.canvas,
          entry.x,
          entry.y,
          entry.w,
          entry.h,
          0,
          -1,
          1,
          1
        );
        if (this.imageLayer) {
          const position = this.getCanvasPosition(entry.x, entry.y);
          this.imageLayer.appendImage({
            imgData,
            left: position[0],
            top: position[1],
            width: w,
            height: h,
          });
        }
        ctx.restore();
      }
    },
    paintSolidColorImageMask: function CanvasGraphics_paintSolidColorImageMask() {
      if (!this.contentVisible) {
        return;