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
scaleY = 1;
      if (drawnWidth > MAX_GROUP_SIZE) {
        scaleX = drawnWidth / MAX_GROUP_SIZE;
        drawnWidth = MAX_GROUP_SIZE;
      }
      if (drawnHeight > MAX_GROUP_SIZE) {
        scaleY = drawnHeight / MAX_GROUP_SIZE;
        drawnHeight = MAX_GROUP_SIZE;
      }
      let cacheId = "groupAt" + this.groupLevel;
      if (group.smask) {
        
        cacheId += "_smask_" + (this.smaskCounter++ % 2);
      }
      const scratchCanvas = this.cachedCanvases.getCanvas(
        cacheId,
        drawnWidth,
        drawnHeight,
        true
      );
      const groupCtx = scratchCanvas.context;
      
      
      groupCtx.scale(1 / scaleX, 1 / scaleY);
      groupCtx.translate(-offsetX, -offsetY);
      groupCtx.transform.apply(groupCtx, currentTransform);
      if (group.smask) {
        
        this.smaskStack.push({
          canvas: scratchCanvas.canvas,