Zooming worked when using a touchpad, touchscreen or discrete mouse;
it was broken for smooth-scrolling mice that emit fractional values.

This makes the code work for all devices.
Tested with:
- A touchscreen
- A multitouch trackpad
- A Logitech MX510 traditional discrete scroll mouse
- A Logitech Anywhere MX hybrid discrete + smooth scroll mouse
- A Microsoft Comfort Mouse 4500 smooth scroll mouse