atlas-texture: Keep reference on potential destination atlas

When a new CoglAtlasTexture tries to fit into an existing CoglAtlas
it should make sure the atlas stays valid while it expands.

https://bugzilla.gnome.org/show_bug.cgi?id=728064

Reviewed-by: Neil Roberts <neil@linux.intel.com>
(cherry picked from commit 2eec9758f67e9073371c2edd63379324849373c4)