WaylandBackend: prevent crash after closing window

[MithicSpirit]

This addresses two related issues around closing windows in the Wayland backend.

1. "Zombie windows":

   When the last gamescope client closes, the wayland backend tries to
   close the window until a new client appears. However, this is done in
   the destructor for the connector for the window, which is stored in
   a shared_ptr. This shared_ptr was held by both steamcompmgr (which
   correctly dropped it) and by the wayland backend, which would never drop
   it. This led to the first window being kept around as a "zombie window",
   which would show the last frame from the previous client, but otherwise
   never update (the backend would only hold a reference to the first
   window, and no subsequent ones).

   This fixes this issue by downgrading the shared_ptr held by the backend
   into a weak_ptr. Thus, the only owning reference to the backend at
   (almost) any given point is held by steamcompmgr, which allows the
   window to be correctly closed when steamcompmgr drops it. This also
   allows for the connector held by the wayland backend to be updated when
   a new window is created.

   HACK: This is NOT memory-safe, and will result in a segmentation fault
   if the connector held by the wayland backend is used (e.g., via
   ::GetCurrentConnector) after it was dropped by steamcompmgr. The code
   already included a TODO to remove the need for the backend to hold
   a reference to the connection, and this is still necessary.

2. Crashing on window close:

   Whenever a window was truly closed (i.e., not left behind as a zombie
   window), gamescope would segfault due to calling
   IsSurfacePlane with null (from Wayland_Pointer_Leave, and maybe a few
   other places). This is addressed by having IsSurfacePlane short-circuit
   if it's passed null.

   HACK: I feel like IsSurfacePlane shouldn't ever be called with a null
   pointer, but this is the easiest way to solve this for now, and the code
   needs refactoring anyway.

See also #1611 and #1456.

[layercak3]

I tested it briefly yesterday and there was issues with the output resolution not being reset, and one time I saw the window show up without an app ID (I have server side decorations with the title bar configured to show the app ID next to the title). I think it should not be closed at least until I reserve some time to check more thoroughly.

[MithicSpirit]

I have that configured as well, but I haven't even been looking at that lol. I haven't tested changing resolutions either. For testing, I just run gamescope --backend wayland -- sh -c 'for _ in {1..10}; do vkcube; sleep 0.2; done' and see what happens as I close the windows in different ways (all in sway, so if someone can test it out on different compositors that would be nice).