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
if (!smask.transferMap && webGLContext.isEnabled) {
      const composed = webGLContext.composeSMask({
        layer: layerCtx.canvas,
        mask,
        properties: {
          subtype: smask.subtype,
          backdrop,
        },
      });
      ctx.setTransform(1, 0, 0, 1, 0, 0);
      ctx.drawImage(composed, smask.offsetX, smask.offsetY);
      return;
    }
    genericComposeSMask(
      maskCtx,
      layerCtx,
      mask.width,
      mask.height,
      smask.subtype,
      backdrop,
      smask.transferMap
    );
    ctx.drawImage(mask, 0, 0);
  }
  const LINE_CAP_STYLES = ["butt", "round", "square"];
  const LINE_JOIN_STYLES = ["miter", "round", "bevel"];
  const NORMAL_CLIP = {};
  const EO_CLIP = {};
  CanvasGraphics.prototype = {
    beginDrawing({