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
if (typeof ImageData !== "undefined" && imgData instanceof ImageData) {
      ctx.putImageData(imgData, 0, 0);
      return;
    }
    
    
    
    
    
    
    
    
    
    
    const height = imgData.height,
      width = imgData.width;
    const partialChunkHeight = height % FULL_CHUNK_HEIGHT;
    const fullChunks = (height - partialChunkHeight) / FULL_CHUNK_HEIGHT;
    const totalChunks = partialChunkHeight === 0 ? fullChunks : fullChunks + 1;
    const chunkImgData = ctx.createImageData(width, FULL_CHUNK_HEIGHT);
    let srcPos = 0,
      destPos;
    const src = imgData.data;
    const dest = chunkImgData.data;
    let i, j, thisChunkHeight, elemsInThisChunk;
    let transferMapRed, transferMapGreen, transferMapBlue, transferMapGray;
    if (transferMaps) {