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
return;
      }
      const ctx = this.ctx;
      this.save();
      const pattern = getShadingPatternFromIR(patternIR);
      ctx.fillStyle = pattern.getPattern(ctx, this, true);
      const inv = ctx.mozCurrentTransformInverse;
      if (inv) {
        const canvas = ctx.canvas;
        const width = canvas.width;
        const height = canvas.height;
        const bl = Util.applyTransform([0, 0], inv);
        const br = Util.applyTransform([0, height], inv);
        const ul = Util.applyTransform([width, 0], inv);
        const ur = Util.applyTransform([width, height], inv);
        const x0 = Math.min(bl[0], br[0], ul[0], ur[0]);
        const y0 = Math.min(bl[1], br[1], ul[1], ur[1]);
        const x1 = Math.max(bl[0], br[0], ul[0], ur[0]);
        const y1 = Math.max(bl[1], br[1], ul[1], ur[1]);
        this.ctx.fillRect(x0, y0, x1 - x0, y1 - y0);
      } else {
        
        
        
        
        
        this.ctx.fillRect(-1e10, -1e10, 2e10, 2e10);
      }
      this.restore();
    },