[RFC v2] protocol: Introduce foreign surface protocol

[Bill Spitzak]

Jonas Ådahl wrote:

> There are two clients (A and B) that can communicate via some IPC
> mechanism where A is the main process and B is a render process. B
> renders to a surface and A decides its visibility and position. Below
> follows an example how foreign surfaces can be used.
> 
> 1. Both A and B connects do the server
> 5. A creates a main surface
> 2. B creates a render surface
> 3. B exports the render surface and retrieves a handle
> 4. B sends the surface handle via IPC to A
> 6. A receives the surface handle via IPC from B
> 7. A creates a foreign wl_surface via wl_foreign_surface_manager
> 8. A creates a wl_subsurface via the wl_subcompositor

I would really like to see A create the surface and pass the handle to 
B. The reason is so that argv can be used as the "IPC" if A does a 
fork+exec of B.