Don't call gdk_cairo_create() outside of begin/end_paint: Bug #732118

Interactive pan/zoom would paint directly to GDK window's Cairo
context outside of a draw() signal.  A recent change to GTK+ in 3.13.3
removes this functionality and requires wrapping this code between
gdk_window_begin_paint_region() and gdk_window_end_paint().  It's far
easier to simply queue a redraw and let the usual draw handler deal
with it.

See:
https://mail.gnome.org/archives/gtk-devel-list/2014-June/msg00010.html