Scrolling volume control should snap to 100%

[kdwk]

When using mouse/ touchpad to control volume by scrolling anywhere on the video, it is easy to make the volume percentage an odd number. If the user then uses the same mechanism to raise the the volume, it will jump from 99% to 101%. I suggest to snap it to 100% first, then only increase it past 100% if the user continues to scroll.

[Rafostar]

Partially related to #395, but I myself vote for implementing snap here instead of more config options.