Fix inverted horizontal scroll on macOS

In winit the swipe from left to right on touchpad should
generate positive horizontal delta change, however on
macOS it was the other way around without
natural scrolling.

This commit inverses the horizontal scrolling delta
in 'MouseScrollDelta' events to match other platforms.

Fixes #1695.

src/platform_impl/macos/view.rs

@@ -1003,7 +1003,8 @@ extern "C" fn scroll_wheel(this: &Object, _sel: Sel, event: id) {
         let state = &mut *(state_ptr as *mut ViewState);
 
         let delta = {
-            let (x, y) = (event.scrollingDeltaX(), event.scrollingDeltaY());
+            // macOS horizontal sign convention is the inverse of winit.
+            let (x, y) = (event.scrollingDeltaX() * -1.0, event.scrollingDeltaY());
             if event.hasPreciseScrollingDeltas() == YES {
                 let delta = LogicalPosition::new(x, y).to_physical(state.get_scale_factor());
                 MouseScrollDelta::PixelDelta(delta)