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
args[j + 2],
              args[j + 3]
            );
            x = args[j + 2];
            y = args[j + 3];
            j += 4;
            break;
          case OPS.curveTo3:
            x = args[j + 2];
            y = args[j + 3];
            ctx.bezierCurveTo(args[j], args[j + 1], x, y, x, y);
            j += 4;
            break;
          case OPS.closePath:
            ctx.closePath();
            break;
        }
      }
      current.setCurrentPoint(x, y);
    },
    closePath: function CanvasGraphics_closePath() {
      this.ctx.closePath();
    },
    stroke: function CanvasGraphics_stroke(consumePath) {
      consumePath = typeof consumePath !== "undefined" ? consumePath : true;
      const ctx = this.ctx;
      const strokeColor = this.current.strokeColor;
      
      
      ctx.globalAlpha = this.current.strokeAlpha;