[BUG] ScrollView not scrolling down

**Describe the bug**
When using a mouse, the ScrollView does not scroll down, only up. When the mouse is scrolled down, nothing happens.
The scroll does work using the arrow keys.

**To Reproduce**
Run the [scroll.rs example](https://github.com/gyscos/cursive/blob/main/cursive/examples/scroll.rs)

**Expected behavior**
For the box to scroll down.

**Environment**
* Operating system used: macOS Monterey
* Backend used: ncurses
* Current locale: en_US.UTF-8
* Cursive version: 0.20.0 from crates.io