xcursor: catch theme inheritance loops

As of currently, when an xcursor theme depends on itself or another theme
that will eventually depend on it, `xcursor_load_theme` will recurse
infinitely while processing the inherits.

This change introduces a stack-allocated linked list of visited nodes
by name, and skips any already visited nodes in the inherit list.

Side effects:
* Since the linked list is stack-allocated, there is a potential for an
  overflow if there is a very long list of dependencies. If this turns out
  to be a legitimate concern, the linked list is trivial to convert to
  being heap-allocated.
* There is an existing linked list (technically doubly linked list)
  implementation in the wayland codebase. As of currently, the xcursor
  codebase does not refer to it. Consequently, this change writes a
  minimal single linked list implementation to utilize directly.

This changeset fixes #317.

Signed-off-by: Chloé Vulquin <toast@bunkerlabs.net>