Reverse zoom control so -> zooms in, <- zooms out
Reverse zoom control so -> zooms in, <- zooms out

Previously, it was the reverse: -> zoomed out, and
<- zoomed in.

The new way seems more intuitive - by moving the
slider to the right, you are increasing the zoom,
which causes the scene to become closer.

jstrait/webgl-city,jstrait/webgl-city