Debug renderViewport() rendering to fix blank Layout.Typeset

add options.throttle-layout=false for Pager.renderViewport()
* default=true
* was not rendering viewport pages due to throttle
add PagerView.helper_IsFetched() to scrollIntoView() instead of collection.fetch()
* add Pager.$el 'xhr-fetching' do disable scrollSpy for renderViewport()
renderViewport() will throttle '.body .page.throttle-layout'