Attach (recommended) or Link to PDF file here:
[Test file.pdf](https://github.com/mozilla/pdf.js/files/4656226/Test.file.pdf)

Configuration:
- Web browser and its version: Chrome Version 81.0.4044.138 (Official Build) (64-bit)
- Operating system and its version: Windows 10
- PDF.js version: latest
- Is a browser extension: no

Steps to reproduce the problem:
1. Open [https://mozilla.github.io/pdf.js/web/viewer.html](https://mozilla.github.io/pdf.js/web/viewer.html)
2. Load the provided .pdf file

What is the expected behavior? (add screenshot)
![image](https://user-images.githubusercontent.com/5569382/82438616-02c3fe80-9aa2-11ea-9003-b75ea1e7aac6.png)

What went wrong? (add screenshot)
![image](https://user-images.githubusercontent.com/5569382/82438640-09527600-9aa2-11ea-810b-071da7e37896.png)


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);
      if (hasBackdrop) {
        composeSMaskBackdrop(maskData.data, r0, g0, b0);
      }
      composeFn(maskData.data, layerData.data, transferMap);
      maskCtx.putImageData(layerData, 0, row);
    }
  }
  function composeSMask(ctx, smask, layerCtx, webGLContext) {
    const mask = smask.canvas;
    const maskCtx = smask.context;
    ctx.setTransform(
      smask.scaleX,
      0,
      0,
      smask.scaleY,
      smask.offsetX,
      smask.offsetY
    );
    const backdrop = smask.backdrop || null;