Linux: fix desktop close/terminate lifecycle handling (#1120)

This avoids a strange case where Linux (both X11 and wayland but
in different ways) failed to properly shut down cleanly.

* Fix X11 close handling by removing destroyed windows from the map,
  which helps avoid delivering duplicate window closed events.
* Restructure how close handling happens on Wayland too, and allow
  the app to respond to a close request just like other platforms.
* Add a Linux second-signal hard exit so that repeated Ctrl+C
  or `kill` comands can actually terminate a failed/hung shutdown.
* Also a tiny fix to windows too: clear Win32 `GWLP_USERDATA` during
  `WM_DESTROY` to avoid accessing an old window pointer.