Adds CoglAtlasSet and CoglAtlas apis

This enables applications to more closely manage their own texture atlases
via a CoglAtlasSet api. A CoglAtlasSet represents a set of CoglAtlases
and a CoglAtlas represents one large texture that is subdivided into
smaller "allocations".

The intention is to enable functionality like cogl-pango to be
implemented outside of Cogl; specifically to allow Rig to handle its own
glyph caches for text rendering.