When iterating over the rectangle map a stack is used to implement a
recursive algorithm. Previously this was slice allocating a linked
list. Now it uses a GArray which is retained with the rectangle map to
avoid frequent allocations which is a little bit faster.
Previously the remaining space was managed as part of the
CoglRectangleMap struct. Now it is stored per node so that at any
point in the hierarchy we can quickly determine how much space is
remaining in all of the node's children. That way when adding a
rectangle we can miss out entire branches more quickly if we know that
there is no way the new rectangle would fit in that branch.
This also adds a function to recursively verify the cached state in
the nodes such as the remaining space and the number of
rectangles. This function is only called when the dump-atlas-image
debug flag is set because it is potentially quite slow.
This simply renames CoglAtlas to CoglRectangleMap without making any
functional changes. The old 'CoglAtlas' is just a data structure for
managing unused areas of a rectangle and it doesn't neccessarily have
to be used for an atlas so it wasn't a very good name.
