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)

if (this._spreadMode === mode) {
      return; 
    }
    if (!isValidSpreadMode(mode)) {
      throw new Error(`Invalid spread mode: ${mode}`);
    }
    this._spreadMode = mode;
    this.eventBus.dispatch("spreadmodechanged", { source: this, mode });
    this._updateSpreadMode( this._currentPageNumber);
  }
  _updateSpreadMode(pageNumber = null) {
    if (!this.pdfDocument) {
      return;
    }
    const viewer = this.viewer,
      pages = this._pages;
    
    viewer.textContent = "";
    if (this._spreadMode === SpreadMode.NONE) {
      for (let i = 0, iMax = pages.length; i < iMax; ++i) {
        viewer.appendChild(pages[i].div);
      }
    } else {
      const parity = this._spreadMode - 1;
      let spread = null;
      for (let i = 0, iMax = pages.length; i < iMax; ++i) {