add mouse xbutton (#24)

Assets/UnityRawInput/Runtime/RawInput.cs

@@ -128,9 +128,11 @@ private static int HandleMouseProc (int code, IntPtr wParam, IntPtr lParam)
             if (state == RawMouseState.LeftButtonDown) HandleKeyDown(RawKey.LeftButton);
             else if (state == RawMouseState.MiddleButtonDown) HandleKeyDown(RawKey.MiddleButton);
             else if (state == RawMouseState.RightButtonDown) HandleKeyDown(RawKey.RightButton);
+            else if (state == RawMouseState.ExtraButtonDown) HandleKeyDown(RawKey.ExtraButton1);
             else if (state == RawMouseState.LeftButtonUp) HandleKeyUp(RawKey.LeftButton);
             else if (state == RawMouseState.MiddleButtonUp) HandleKeyUp(RawKey.MiddleButton);
             else if (state == RawMouseState.RightButtonUp) HandleKeyUp(RawKey.RightButton);
+            else if (state == RawMouseState.ExtraButtonUp) HandleKeyUp(RawKey.ExtraButton1);
             else return Win32API.CallNextHookEx(IntPtr.Zero, code, wParam, lParam);
             return InterceptMessages ? 1 : Win32API.CallNextHookEx(IntPtr.Zero, 0, wParam, lParam);
         }

Assets/UnityRawInput/Runtime/RawKey.cs

@@ -199,4 +199,6 @@ public enum RawKey : ushort
         PA1 = 0xFD,
         OEMClear = 0xFE
     }
+
+    // https://docs.microsoft.com/en-us/windows/win32/inputdev/virtual-key-codes
 }

Assets/UnityRawInput/Runtime/RawMouseState.cs

@@ -1,6 +1,6 @@
 namespace UnityRawInput
 {
-    public enum RawMouseState
+    public enum RawMouseState : ushort
     {
         MouseMove = 0x0200,
         LeftButtonDown = 0x0201,
@@ -9,6 +9,10 @@ public enum RawMouseState
         MiddleButtonDown = 0x0207,
         MiddleButtonUp = 0x0208,
         RightButtonDown = 0x0204,
-        RightButtonUp = 0x0205
+        RightButtonUp = 0x0205,
+        ExtraButtonDown = 0x20B,
+        ExtraButtonUp = 0x20C
     }
+
+    // https://www.pinvoke.net/default.aspx/Constants/WM.html
 }