[cairo] changed semantics of cairo_destroy

[Maarten Bosmans]

Last week I chased down a bug that was causing rendering artifacts in
Gtk+ when used with a recent Cairo snapshot. It bisected to this
commit in Cairo:
http://cgit.freedesktop.org/cairo/commit/?id=4438cc6a49e7e902dce045706f7125a2c3e2174b
(Gtk bug report is here: https://bugzilla.gnome.org/show_bug.cgi?id=628291)

I think this change warrants an explicit notice in the release notes
for 1.10, as it is certainly something for Cairo users to be aware of
and I didn't see the change announced in one of the 1.9.x snapshot
notes.

By the way, I proposed a patch in Gtk+ to overcome this problem by adding a
    cairo_surface_flush (cairo_get_target (cr));
before every
    cairo_destroy (cr);
where relevant. Is this the correct approach (it certainly resolves
the bug I was seeing), or should there be something done with
cairo_device_t?

Maarten