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)


if (!smask.transferMap && webGLContext.isEnabled) {
      const composed = webGLContext.composeSMask({
        layer: layerCtx.canvas,
        mask,
        properties: {
          subtype: smask.subtype,
          backdrop,
        },
      });
      ctx.setTransform(1, 0, 0, 1, 0, 0);
      ctx.drawImage(composed, smask.offsetX, smask.offsetY);
      return;
    }
    genericComposeSMask(
      maskCtx,
      layerCtx,
      mask.width,
      mask.height,
      smask.subtype,
      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 = {};
  CanvasGraphics.prototype = {
    beginDrawing({