xwayland: Update screen pixmap for root window in xwl_window_set_pixmap
If the old window pixmap was the screen pixmap.
Fixes screen->GetScreenPixmap() returning a stale pointer to a destroyed pixmap with rootful Xwayland. It would result in a crash after resizing the Xwayland window, or at the latest when shutting down.
Fixes: 6779ec5b ("xwayland: Use window pixmap as a window buffer") Closes: #1621 (closed)
Edited by Michel Dänzer