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)

for (let i = 0, ii = this._pages.length; i < ii; i++) {
      if (
        this._pages[i] &&
        this._pages[i].renderingState !== RenderingStates.FINISHED
      ) {
        this._pages[i].reset();
      }
    }
  }
  /**
   * @private
   */
  _cancelRendering() {
    for (let i = 0, ii = this._pages.length; i < ii; i++) {
      if (this._pages[i]) {
        this._pages[i].cancelRendering();
      }
    }
  }
  /**
   * @param {PDFPageView} pageView
   * @returns {Promise} Returns a promise containing a {PDFPageProxy} object.
   * @private
   */
  _ensurePdfPageLoaded(pageView) {