libview: Fix the calculation of the page area in non continuous mode

Take borders into account when calculating the x coordinate of the page
area.
libview: Use the right border when transforming doc rect ot view rect

Use top instead of right border for the y coordinate.
libview: Take border into account when converting view points to doc points
libview: Make sure we don't return negative values when converting view points to doc points
libview: Do not use the spacing when setting the vertical scroll value in continuous mode

The y value received in scroll_to_point already contains the spacings.
libview: Keep the scroll position when switching between continuous and non continuous mode

https://bugzilla.gnome.org/show_bug.cgi?id=554843