shared/cairo-util: Hold onto our pattern reference until we're done

This doesn't actually fix a bug - cairo refcounts this. But I
really don't like the look of code that drops a reference then
continues to use it.

While we're here, set a different pattern when we're done so the
one we allocated loses its last reference.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>