bugfix: motion events with absolute coordinates are now merged correctly

        Problem: On merge of events with absolute coordinates the
                 coordinates of previous events are just overwritten. When
                 merging a sequence of X and Y motion events, the x-component
                 is ignored as it is overwritten by the y-event.

        Solution: Initialize absolute coordinates with invalid value -1. Check
                  if a coordiante is invalid before overwriting.