mi: decouple miCreateScreenResources from pScreen->{width,height}

This allows pScreen->{width,height} to be changed to smaller values after miScreenInit() is called but before pScreen is further set up / used by dix, so that the screen size can be fit to the mode of choice even when it does not have the maximum width and/or height among its peers.

XGetImage() for one will fail once the screen is change back to a bigger size with xrandr, which prevents e.g. x11vnc to work on the server.

Also see !848.