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)

return new StructTreeLayerBuilder({
      pdfPage,
    });
  }
  /**
   * @type {boolean} Whether all pages of the PDF document have identical
   *   widths and heights.
   */
  get hasEqualPageSizes() {
    const firstPageView = this._pages[0];
    for (let i = 1, ii = this._pages.length; i < ii; ++i) {
      const pageView = this._pages[i];
      if (
        pageView.width !== firstPageView.width ||
        pageView.height !== firstPageView.height
      ) {
        return false;
      }
    }
    return true;
  }
  /**
   * Returns sizes of the pages.
   * @returns {Array} Array of objects with width/height/rotation fields.
   */
  getPagesOverview() {