server: Rework how XID object assignment is done
server: Rework how XID object assignment is done

Right now, all XID assignment is done by the _createWindowInternal or
_createPixmapInternal functions. Which, while great for our internal
server implementations, doesn't work too well for redirected draw trees
where we have dynamically reconfigured pixmaps that we have to give
XIDs to.

Make it a bit easier to just say "new ServerPixmap" and DTRT.

magcius/xplain,magcius/xplain