xcb: Remove free XID cache
cairo-xcb kept a cache of free xid to avoid calling xcb_generate_id() later. However, this is unsafe: When libxcb runs out of ids, it asks the X11 server for an empty range of ids to use. The X11 server of course does not know about cairo's cache and could hand out an id that cairo will use again later. This would then result in BadIdChoice errors later.
Fix this by simply removing the whole cache.
Fixes: #434 (closed) Signed-off-by: Uli Schlachter psychon@znc.in