Properly release the seat on RDP disconnect. Using current master
branch which is commit d93c0f7059 ("backend-rdp: fix memory leak")
I was not able to reproduce the crash on reconnect as mentioned in the
current comment. Using Weston with rdp-backend directly as well as
using the screen-share plug-in allowed to reconnect just fine. Hence
release the Weston seat properly using weston_seat_release and free
the seat structure. This also avoids mouse pointers displayed for
every RDP connection.
Signed-off-by: Stefan Agner <stefan@agner.ch>