Add rails to input wheel events.

Source/platform/PlatformEvent.h

Index: Source/platform/PlatformEvent.h
diff --git a/Source/platform/PlatformEvent.h b/Source/platform/PlatformEvent.h
index c47f2a6702e9d718f0b9a8697745f0a41c59297f..ca813eec329ea07da3d0cf4a0af3c9b5f636f0fa 100644
--- a/Source/platform/PlatformEvent.h
+++ b/Source/platform/PlatformEvent.h
@@ -83,6 +83,12 @@ public:
         RightButtonDown  = 1 << 8,
     };
 
+    enum RailsMode {
+        RailsModeFree       = 0,
+        RailsModeHorizontal = 1,
+        RailsModeVertical   = 2,
+    };
+
     Type type() const { return static_cast<Type>(m_type); }
 
     bool shiftKey() const { return m_modifiers & ShiftKey; }