evdev: Keep track of the pointer coordinate ourself

When multiple relative motion events are received and queued, we can't
base the relative => absolute motion conversion off of the stage pointer
position, since that is only updated when the queue is processed at
the beginning of each frame. The effect of trying to use the stage
pointer position was that subsequent motion events were effectively
dropped.

To improve things, switch to keeping track of the pointer position
ourselves in the evdev backend and adding to that.

This has the side effect of making the internal function
_clutter_input_device_set_coords not effect the internal coordinate
state of the evdev stage, but AFAICS there is nothing depending on that
so that should be fine.

https://bugzilla.gnome.org/show_bug.cgi?id=746328