Create a multi-surface state synchronization protocol

While surfaces in a subsurface tree can commit their state together atomically, other surfaces cannot.

A use case where this is important is when repositioning a xdg_popup. In a early version of the xdg-shell popup repositioning protocol, a inter-xdg-surface synchronization method was added, but eventually scrapped in favor of exploring a more plumbing level surface state synchronization framework.

A quick sketch of how it may look:

<?xml version="1.0" encoding="UTF-8"?>
<protocol name="sync">
 
  <interface name="zwp_sync_manager_v1" version="1">
    <request name="destroy" type="destructor"/>
 
    <request name="create_sync_point">
      <arg name="id" type="new_id" interface="zwp_sync_point_v1"/>
    </request>
  </interface>
 
  <interface name="zwp_sync_point_v1" version="1">
    <request name="destroy" type="destructor"/>
 
    <request name="add_surface">
      <arg name="id" type="object" interface="wl_surface"/>
    </request>
 
    <request name="commit"/>
  </interface>
 
</protocol>