util functions
search.get_onscreen_searchpattern()
fix #38: streak-mode in reverse direction
fix: streak-mode too eager with scrolloff/sidescrolloff

If scrolloff/sidescrolloff are set, additional text is
scrolled into the visible area, which may trigger
streak-mode if there are >=2 addtional matches scrolled
into view.
fix #32: streak-mode only for on-screen matches

streak-mode should _not_ be triggered by matches
that are off-screen in _any_ direction.