Use a setting 'copt_overlap_lines' (default to 1) for the
number of text lines from previous page to show on next
page (instead of a hardcoded and unscaled 20).
Take footer height into account for overlap calculation
and dim area.
Show overlap only when changing pages (not when panning).
Also fix ReaderFooter:getHeight() which was returning
a higher height than the real one since footer refactoring.