DeviceManagerXi2: Update cached core pointer in getter if NULL

XIGetClientPointer() may return the device id '0' when called early.
This patch makes pointer cursors work in nested mutter Wayland
sessions again.

https://bugzilla.gnome.org/show_bug.cgi?id=729462
device-manager-xi2: Fix core pointer retrieval race

The core pointer concept doesn't really exist anymore in an XI2 world,
so the clutter API is a bit of a mismatch with what X provides. Using
XIGetClientPointer doesn't really help, as far as i can tell the
semantics of XIGetClientPointer are essentially: Whatever the X server
picked when it had to reply with device-dependant data to a query
without a device specifier. Not very useful...

To make matters worse, whether XIGetClientPointer returns a valid
pointer depends on whether there has been a query that forced it to pick
one in the first place, making the whole thing pretty non-deterministic.

This patch changes things around such that instead of using
XIGetClientPointer to determine the core pointer, we simply pick the
first master pointer device. In practise this will essentially always
be the X virtual core pointer.

https://bugzilla.gnome.org/show_bug.cgi?id=729462