Only post rel motion events if we have a non-zero delta

We trunc the li_fixed_t to int, so we don't always have a delta to post

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Switch to ValuatorMasks for motion and axis events

Allows us to use subpixel data, not that the server does a good job with it.
But at least for scrolling we should now be able to scroll by  small amounts
too.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>