Re-factor the PresentationMode handling in `BaseViewer.#ensurePageViewVisible`
Re-factor the PresentationMode handling in `BaseViewer.#ensurePageViewVisible`

Given that we're (ab)using spread-modes in order to ensure that pages are centered *vertically* in PresentationMode, this re-factoring simplifies the code slightly.
Furthermore, in the event that we *possibly* want to try and support spread-modes in PresentationMode[1] this re-factoring will also prevent future duplication.

---
[1] Note that I'm not particularly keen on doing that, since documents with varying page sizes will be annoying to handle.

timvandermeij/pdf.js,showpad/mozilla-pdf.js,showpad/mozilla-pdf.js,timvandermeij/pdf.js,Shoobx/pdf.js,Snuffleupagus/pdf.js,timvandermeij/pdf.js,Shoobx/pdf.js,mozilla/pdf.js,nawawi/pdf.js,mozilla/pdf.js,Snuffleupagus/pdf.js,mozilla/pdf.js,nawawi/pdf.js,nawawi/pdf.js,Shoobx/pdf.js,showpad/mozilla-pdf.js,Snuffleupagus/pdf.js