add mouse wheel support

src/Game.cs

@@ -82,6 +82,8 @@ namespace MoonWorks
                 {
                     while (accumulator >= timestep)
                     {
+                        Inputs.Mouse.Wheel = 0;
+
                         HandleSDLEvents();
 
                         Inputs.Update();
@@ -112,6 +114,10 @@ namespace MoonWorks
                     case SDL.SDL_EventType.SDL_TEXTINPUT:
                         HandleTextInput(_event);
                         break;
+
+                    case SDL.SDL_EventType.SDL_MOUSEWHEEL:
+                        Inputs.Mouse.Wheel += _event.wheel.y;
+                        break;
                 }
             }
         }

src/Input/Mouse.cs

@@ -13,6 +13,8 @@ namespace MoonWorks.Input
         public int DeltaX { get; private set; }
         public int DeltaY { get; private set; }
 
+        public int Wheel { get; internal set; }
+
         private bool relativeMode;
         public bool RelativeMode
         {