I was wondering if WebGL is considered stable enough to be enabled by default. It makes some PDFs render much faster and it has been off by default for a while now, so we have had quite some time to test it.

If we decide to enable WebGL by default, we should not forget to update https://github.com/mozilla/pdf.js/blob/master/extensions/chromium/preferences_schema.json#L39 as well.

/cc @yurydelendik 
/cc @Snuffleupagus (because IIRC you were testing WebGL too)

ctx.save();
        ctx.transform(
          scaleX,
          skewX,
          skewY,
          scaleY,
          positions[i],
          positions[i + 1]
        );
        ctx.scale(1, -1);
        ctx.drawImage(maskCanvas.canvas, 0, 0, width, height, 0, -1, 1, 1);
        ctx.restore();
      }
    }
    paintImageMaskXObjectGroup(images) {
      if (!this.contentVisible) {
        return;
      }
      const ctx = this.ctx;
      const fillColor = this.current.fillColor;
      const isPatternFill = this.current.patternFill;
      for (let i = 0, ii = images.length; i < ii; i++) {
        const image = images[i];
        const width = image.width,
          height = image.height;
        const maskCanvas = this.cachedCanvases.getCanvas(
          "maskCanvas",
          width,
          height
        );