xdg-shell: ack_configure must be strictly monotonic

Clients must send ack_configure in a strictly monotonic order wrt received configure events. It is an error to send an ack_configure request for a configure event which was sent prior to the last ack_configure for that surface, or to send multiple ack_configures for the same configure event.

Weston and wlroots already use this interpretation, however Mutter and KWayland are more lax and allow duplicates. This clarification tightens the spec working to explicitly encode the Weston/wlroots behaviour.

Fixes: #21 (closed)

Signed-off-by: Daniel Stone daniels@collabora.com