Fix even-odd page contiguous-ness.

In page view, the pages are shown in contiguous pairs, unconditionally starting with the second page, regardless of page numbering.

This fix makes each even page contiguous with the odd page following it, according to page numbering, i.e. taking into account `_pageNumberOffset`.
Merge pull request #1007 from mgavioli/even-odd-page-distance

Fix even-odd page contiguous-ness.