wayland: Remove absolute position from processMotion() that handles relative moves

API-wise it's odd to require absolute position with relative pointer
moves.

This cleans up some of InputRedirection code by making it simply forward
pointer input events from input devices to the pointer input device
handler, which knows better what to do with the delta value.

The pointer input device handler will compute the absolute position based
on the specified delta value, but it could also do something else, e.g.
not move the pointer at all if it's locked, etc.