backend/wayland: handle WL_EVENT_WRITABLE for Wayland socket
backend/wayland: handle WL_EVENT_WRITABLE for Wayland socket

We need to flush when the connection is writable again. This is important in
case the write buffer becomes full. This is also what Weston does [1].

[1]: https://gitlab.freedesktop.org/wayland/weston/blob/master/libweston/compositor-wayland.c#L2593

swaywm/wlroots,ascent12/wlroots,SirCmpwn/wlroots,ascent12/wlroots,swaywm/wlroots