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
switch (transferMaps.length) {
        case 1:
          transferMapRed = transferMaps[0];
          transferMapGreen = transferMaps[0];
          transferMapBlue = transferMaps[0];
          transferMapGray = transferMaps[0];
          break;
        case 4:
          transferMapRed = transferMaps[0];
          transferMapGreen = transferMaps[1];
          transferMapBlue = transferMaps[2];
          transferMapGray = transferMaps[3];
          break;
      }
    }
    
    
    if (imgData.kind === ImageKind.GRAYSCALE_1BPP) {
      
      const srcLength = src.byteLength;
      const dest32 = new Uint32Array(dest.buffer, 0, dest.byteLength >> 2);
      const dest32DataLength = dest32.length;
      const fullSrcDiff = (width + 7) >> 3;
      let white = 0xffffffff;
      let black = IsLittleEndianCached.value ? 0xff000000 : 0x000000ff;
      if (transferMapGray) {
        if (transferMapGray[0] === 0xff && transferMapGray[0xff] === 0) {
          [white, black] = [black, white];
        }
      }