input-device: Guard against double free

Dispose() may be called more than once, so calling g_free directly
on the device name is unsafe. Instead, use g_clear_pointer() to
make sure we don't attempt to free the memory again.

https://bugzilla.gnome.org/show_bug.cgi?id=719563
ClutterStageCogl: Ignore a clip the size of the stage

If the clip region includes the entire stage, ignore it - we aren't
actually clipped.

https://bugzilla.gnome.org/show_bug.cgi?id=719901