Optimized by not calling compute_border() so many
times in the adjustment-value-changed signal handler.

For that we added new ev_view_get_page_extents_for_border()
function that uses the passed in GtkBorder argument instead
of obtain it internally, calling compute_border().

We use this new function to replace ev_view_get_page_extents()
in the cases it was used inside a loop iterating over several
pages, as those provoked performance bottlenecks when scrolling
pdf's with large number of pages.

Issue #700