croutonwheel: Invert vertical scrolling direction

Handling of input events was changed recently in Chromium OS, and
the default behaviour in Xephyr is now always the opposite of the
one in Chromium OS (setting traditional scrolling in Chromium OS
leads to Australian in Xephyr, and vice-versa).
Merge pull request #1015 from drinkcat/invert-wheel

croutonwheel: Invert vertical scrolling direction