wayland: fix up/down mix up in discrete events

wayland: fix up/down mix up in discrete events

The wayland specification for discrete step information for scroll and
other axes reads:

| The discrete value carries the directional information. e.g. a
| value of -2 is two steps towards the negative direction of this axis.

mutter sets a value of 1 for SCROLL_DOWN events and -1 for SCROLL_UP
events.

gdkdevice Wayland backend does the opposite, it translates a positive
discrete value as SCROLL_UP and a negative value as SCROLL_DOWN, which
ends up inverting the scrolling direction.

Fix the logic in gdkdevice Wayland to use a positive value as
SCROLL_DOWN and a negative value as SCROLL_UP so that it matches mutter
and weston logic.

https://bugzilla.gnome.org/show_bug.cgi?id=765907