input: add support for natural scrolling

Closes #69

src/backends/metal.rs

@@ -296,6 +296,7 @@ struct MetalInputDevice {
     tap_enabled: Cell<Option<bool>>,
     drag_enabled: Cell<Option<bool>>,
     drag_lock_enabled: Cell<Option<bool>>,
+    natural_scrolling_enabled: Cell<Option<bool>>,
 }
 
 #[derive(Clone)]
@@ -354,6 +355,9 @@ impl MetalInputDevice {
         if let Some(enabled) = self.drag_lock_enabled.get() {
             dev.device().set_drag_lock_enabled(enabled);
         }
+        if let Some(enabled) = self.natural_scrolling_enabled.get() {
+            dev.device().set_natural_scrolling_enabled(enabled);
+        }
     }
 
     fn pre_pause(&self) {
@@ -465,6 +469,13 @@ impl InputDevice for MetalInputDevice {
             dev.device().set_drag_lock_enabled(enabled);
         }
     }
+
+    fn set_natural_scrolling_enabled(&self, enabled: bool) {
+        self.natural_scrolling_enabled.set(Some(enabled));
+        if let Some(dev) = self.inputdev.get() {
+            dev.device().set_natural_scrolling_enabled(enabled);
+        }
+    }
 }
 
 impl MetalInputDevice {

src/backends/metal/monitor.rs

@@ -296,6 +296,7 @@ impl MetalBackend {
             tap_enabled: Default::default(),
             drag_enabled: Default::default(),
             drag_lock_enabled: Default::default(),
+            natural_scrolling_enabled: Default::default(),
         });
         slots[slot] = Some(dev.clone());
         self.device_holder

src/backends/x.rs

@@ -1162,6 +1162,10 @@ impl InputDevice for XSeatKeyboard {
     fn set_drag_lock_enabled(&self, enabled: bool) {
         let _ = enabled;
     }
+
+    fn set_natural_scrolling_enabled(&self, enabled: bool) {
+        let _ = enabled;
+    }
 }
 
 impl InputDevice for XSeatMouse {
@@ -1223,4 +1227,8 @@ impl InputDevice for XSeatMouse {
     fn set_drag_lock_enabled(&self, enabled: bool) {
         let _ = enabled;
     }
+
+    fn set_natural_scrolling_enabled(&self, enabled: bool) {
+        let _ = enabled;
+    }
 }