fixed the sliding speed

The universe was moving too rapidly when using the mouse for sliding
gestures.

Reported-by: Shaun Jackman <sjackman@gmail.com>
Reviewed-by: Pier-Luc Plante <pier-luc.plante.1@ulaval.ca>
Signed-off-by: Sébastien Boisvert <sebastien.boisvert.3@ulaval.ca>