porting gdk -> cairo

[Allin Cottrell <cottrell wfu edu>]

I gather from recent discussion on the gtk-devel list that a good
deal of the old GDK drawing API may be removed for GTK 3 in favor
of use of cairo: in particular GdkGC seems likely to disappear.
There appear to be good arguments for this, but it means some
extra work for people trying to get their apps GTK3-ready.

Benjamin Otte has some notes on porting at
http://blogs.gnome.org/otte/2010/07/27/rendering-cleanup/ but
there's one piece of GDK code I've been using for which I can't
figure out the cairo equivalent and I wonder if anyone can help.

The context is zooming to a selected portion of a graphic
displayed in a GTK window. If the user selects a Zoom menu item
she can then drag out a rectangular outline on the current image,
and when the mouse button is released the view snaps to the chosen
rectangle.

The tricky part is avoiding a horrible mess as the user drags the
rectangle. This is achieved via a cycle of draw-display-erase as
the mouse pointer moves. In GDK, the "erase" part is done by
redrawing the last-shown rectangle using an inverted GdkGC:

gdk_gc_set_function(gc, GDK_INVERT);

How would you do this in cairo? Thanks.

-- 
Allin Cottrell
Department of Economics
Wake Forest University