dix: updated enter/leave core event model.

As proposed by Owen Taylor [1], the enter-leave event model needs to adjust
the events sent to each window depending on the presence of pointers in a
window, or in a subwindow.

The new model can be summarised as:
- if the pointer moves into or out of a window that has a pointer in a child
  window, the events are modified to appear as if the pointer was moved out of
  or into this child window.
- if the pointer moves into or out of a window that has a pointer in a parent
  window, the events are modified to appear as if the pointer was moved out of
  or into this parent window.

Note that this model requires CoreEnterLeaveEvent and DeviceEnterLeaveEvent to
be split and treated separately.

[1] http://lists.freedesktop.org/archives/xorg/2008-August/037606.html

Signed-off-by: Peter Hutterer <peter.hutterer@redhat.com>

dix/enterleave.h

@@ -63,4 +63,8 @@ extern void DeviceEnterLeaveEvent(DeviceIntPtr mouse,
                                   WindowPtr pWin,
                                   Window child);
 
+extern void EnterWindow(DeviceIntPtr dev,
+                        WindowPtr win,
+                        int mode);
+
 #endif /* _ENTERLEAVE_H_ */