Resolve "Mouse scrolling are laggy with big pdf (continuous mode)"

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

For that we modified ev_view_get_page_extents() to add new gboolean parameter saying if we should use the passed in &border struct as a valid one or otherwise obtain it as before, calling compute_border().

We replace all occurences of ev_view_get_page_extents() with the new parameter, setting it to TRUE and passing a valid &border in the cases it was used inside a loop iterating over several pages.

Issue #700 (closed)

Closes #700 (closed)