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)

const activeSMask = this.current.activeSMask;
      const drawnWidth = activeSMask.canvas.width;
      const drawnHeight = activeSMask.canvas.height;
      const cacheId = "smaskGroupAt" + this.groupLevel;
      const scratchCanvas = this.cachedCanvases.getCanvas(
        cacheId,
        drawnWidth,
        drawnHeight,
        true
      );
      const currentCtx = this.ctx;
      const currentTransform = currentCtx.mozCurrentTransform;
      this.ctx.save();
      const groupCtx = scratchCanvas.context;
      groupCtx.scale(1 / activeSMask.scaleX, 1 / activeSMask.scaleY);
      groupCtx.translate(-activeSMask.offsetX, -activeSMask.offsetY);
      groupCtx.transform.apply(groupCtx, currentTransform);
      activeSMask.startTransformInverse = groupCtx.mozCurrentTransformInverse;
      copyCtxState(currentCtx, groupCtx);
      this.ctx = groupCtx;
      this.setGState([
        ["BM", "source-over"],
        ["ca", 1],
        ["CA", 1],
      ]);
      this.groupStack.push(currentCtx);
      this.groupLevel++;
    }
    suspendSMaskGroup() {