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 image = images[i];
        const width = image.width,
          height = image.height;
        const maskCanvas = this.cachedCanvases.getCanvas(
          "maskCanvas",
          width,
          height
        );
        const maskCtx = maskCanvas.context;
        maskCtx.save();
        putBinaryImageMask(maskCtx, image);
        maskCtx.globalCompositeOperation = "source-in";
        maskCtx.fillStyle = isPatternFill
          ? fillColor.getPattern(maskCtx, this)
          : fillColor;
        maskCtx.fillRect(0, 0, width, height);
        maskCtx.restore();
        ctx.save();
        ctx.transform.apply(ctx, image.transform);
        ctx.scale(1, -1);
        ctx.drawImage(maskCanvas.canvas, 0, 0, width, height, 0, -1, 1, 1);
        ctx.restore();
      }
    },
    paintImageXObject: function CanvasGraphics_paintImageXObject(objId) {
      if (!this.contentVisible) {
        return;
      }
      const imgData = objId.startsWith("g_")
        ? this.commonObjs.get(objId)