backends/x11: explicitly free the outputs

...before releasing the connection!

this was randomly causing crashes because of use-after-disconnect
problems. since we would tear down the connection in ~X11WindowedBackend
but outputs wouldn't get cleaned up until QObject children cleanup of
the X11WindowedBackend object (or more precisely the OutputBase). this
would then result in ~X11WindowedOutput accessing a backend connection
that had already been closed

to help debug this type of problem moving forward let's also reset the
connection to nullptr during destruction. it's kinda pointless but it
makes it easier to spot use-after-disconnect

BUG: 466183


(cherry picked from commit 4fcc5456)