Always have the correct viewport when zooming

Whenever the scrollbar value changes we request a new pixmap so we
always have an up to date viewport.
Unfortunately that can lead to unnecessary calls when zooming. That's
because the zoom event changes the value of each scrollbar but not all
at once. Instead one scrollbar value is changed after the other (leading
to two requests).
The problem here is that when the first request is made just one of the
scrollbars have its updated value while the other still carries the old
one. Previously that wasn't a big deal but now we depend on the correct
scrollbar values to get the visible viewport and thus request only the
visible tiles (and its whereabouts).
Without that change we're making requests to tiles that are not actually
visible and this only gets worse as the zoom level gets higher.