Fix a regression in the implementation of `reflowableView.insureElementVisibility`
Fix a regression in the implementation of `reflowableView.insureElementVisibility`

Fixes #416

readium/readium-shared-js,NYPL-Simplified/readium-shared-js,NYPL/readium-shared-js,readium/readium-shared-js,NYPL-Simplified/readium-shared-js,NYPL/readium-shared-js