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 groupCtx = this.ctx;
      this.groupLevel--;
      this.ctx = this.groupStack.pop();
      composeSMask(
        this.ctx,
        this.current.activeSMask,
        groupCtx,
        this.webGLContext
      );
      this.ctx.restore();
      this.ctx.save(); 
      copyCtxState(groupCtx, this.ctx);
      
      this.current.resumeSMaskCtx = groupCtx;
      
      
      const deltaTransform = Util.transform(
        this.current.activeSMask.startTransformInverse,
        groupCtx.mozCurrentTransform
      );
      this.ctx.transform.apply(this.ctx, deltaTransform);
      
      groupCtx.save();
      groupCtx.setTransform(1, 0, 0, 1, 0, 0);
      groupCtx.clearRect(0, 0, groupCtx.canvas.width, groupCtx.canvas.height);
      groupCtx.restore();
    }
    resumeSMaskGroup() {