Attach (recommended) or Link to PDF file here:
https://s3.eu-west-1.amazonaws.com/scai-dev.website.com/tmp/8ab8b56a-fa91-11eb-8d74-0242ac110004.pdf

Hi,
Please see an incorrect canvas rendering for the following document - page 9 specifically. It reproduces on https://mozilla.github.io/pdf.js/web/viewer.html (tried with Chrome). Other pages in this document look pretty much the same but the problem is on page 9 only.

Original screenshot:
![image](https://user-images.githubusercontent.com/23075641/129066656-7bb2fb56-2b88-4f40-8e67-b95bdeb9e521.png)


PDF JS:

![image](https://user-images.githubusercontent.com/23075641/129066734-56503a5c-413c-49c1-9e1c-470da3a642ed.png)

maskCtx,
  layerCtx,
  width,
  height,
  subtype,
  backdrop,
  transferMap
) {
  const hasBackdrop = !!backdrop;
  const r0 = hasBackdrop ? backdrop[0] : 0;
  const g0 = hasBackdrop ? backdrop[1] : 0;
  const b0 = hasBackdrop ? backdrop[2] : 0;
  let composeFn;
  if (subtype === "Luminosity") {
    composeFn = composeSMaskLuminosity;
  } else {
    composeFn = composeSMaskAlpha;
  }
  
  const PIXELS_TO_PROCESS = 1048576;
  const chunkSize = Math.min(height, Math.ceil(PIXELS_TO_PROCESS / width));
  for (let row = 0; row < height; row += chunkSize) {
    const chunkHeight = Math.min(chunkSize, height - row);
    const maskData = maskCtx.getImageData(0, row, width, chunkHeight);
    const layerData = layerCtx.getImageData(0, row, width, chunkHeight);