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
lly,
      urx,
      ury
    ) {
      
      
      this.ctx.rect(llx, lly, urx - llx, ury - lly);
      this.clip();
      this.endPath();
    },
    
    getColorN_Pattern: function CanvasGraphics_getColorN_Pattern(IR) {
      let pattern;
      if (IR[0] === "TilingPattern") {
        const color = IR[1];
        const baseTransform =
          this.baseTransform || this.ctx.mozCurrentTransform.slice();
        const canvasGraphicsFactory = {
          createCanvasGraphics: ctx => {
            return new CanvasGraphics(
              ctx,
              this.commonObjs,
              this.objs,
              this.canvasFactory,
              this.webGLContext
            );
          },
        };
        pattern = new TilingPattern(
          IR,