Add doubly linked lists

Add doubly linked lists

This commit adds some new inline functions to maintain a doubly linked
list.

The way to use them is to embed a pixman_link_t into the structures
that should be linked, and use a pixman_list_t as the head of the
list.

The new functions are

    pixman_list_init (pixman_list_t *list);
    pixman_list_prepend (pixman_list_t *list, pixman_link_t *link);
    pixman_list_move_to_front (pixman_list_t *list, pixman_link_t *link);

There are also a new macro:

    CONTAINER_OF(type, member, data);

that can be used to get from a pointer to a member to the containing
structure.

V2: Use the C89 macro offsetof() instead of rolling our own -
suggested by Alan Coopersmith.