Attach (recommended) or Link to PDF file here:
[backgroundimage_not_visible_anonymisiert.pdf](https://github.com/mozilla/pdf.js/files/6544725/backgroundimage_not_visible_anonymisiert.pdf)


Configuration:
- Web browser and its version: Chrome Version 90.0.4430.212 (Offizieller Build) (x86_64)
- Operating system and its version: Windows 10, MacOs BigSur
- PDF.js version: 2.5.207
- Is a browser extension: no

Steps to reproduce the problem:
1. open the document in demo viewer.js
2.

What is the expected behavior? (add screenshot)
The backgroundimage gets rendered like in Acrobat Reader
<img width="1348" alt="Bildschirmfoto 2021-05-26 um 09 44 21" src="https://user-images.githubusercontent.com/1286150/119621970-00a82480-be07-11eb-8fed-f74ac04b840b.png">

What went wrong? (add screenshot)
Backgroundimage is not rendered only some  text
<img width="987" alt="Bildschirmfoto 2021-05-26 um 09 43 57" src="https://user-images.githubusercontent.com/1286150/119621938-fab24380-be06-11eb-92ac-4ecb2c9334b1.png">




Link to a viewer (if hosted on a site other than mozilla.github.io/pdf.js or as Firefox/Chrome extension):

const mask = smask.canvas;
    const maskCtx = smask.context;
    ctx.setTransform(
      smask.scaleX,
      0,
      0,
      smask.scaleY,
      smask.offsetX,
      smask.offsetY
    );
    genericComposeSMask(
      maskCtx,
      layerCtx,
      mask.width,
      mask.height,
      smask.subtype,
      smask.backdrop,
      smask.transferMap
    );
    ctx.drawImage(mask, 0, 0);
  }
  const LINE_CAP_STYLES = ["butt", "round", "square"];
  const LINE_JOIN_STYLES = ["miter", "round", "bevel"];
  const NORMAL_CLIP = {};
  const EO_CLIP = {};
  
  class CanvasGraphics {
    constructor(