Fix wrong page being displayed when entering/exiting Presentation mode with zoom set to 'page-fit'

The only way to solve this seems to revert to the earlier behaviour, where `PDFView.getVisiblePages` doesn't actually check which page is visible whilst Presentation mode is active, but instead just assumes that the current page is visible.

Fixes #4232.

(If it turns out that there are still issues even after this PR, I think we would need to back out #4074 as well.)