Fix generation of Leave events when using tablet devices

When both mouse and tablet events are handled by QWindowsPointerHandler,
m_currentWindow variable is shared among the two event streams, therefore
each stream should ensure it does equivalent operations, when changing it.

Here we should subscribe to the Leave events, when we emit Enter event
from the inside of the tablet events flow. Without whis subscription,
the cursor may stuck in "resize" state when crossing the window's
frame multiple times.

Change-Id: I88df4a42ae86243e10ecd4a4cedf87639c96d169
Reviewed-by: Andre de la Rocha <andre.rocha@qt.io>

src/plugins/platforms/windows/qwindowspointerhandler.cpp

@@ -614,6 +614,9 @@ bool QWindowsPointerHandler::translatePenEvent(QWindow *window, HWND hwnd, QtWin
         if (m_needsEnterOnPointerUpdate) {
             m_needsEnterOnPointerUpdate = false;
             if (window != m_currentWindow) {
+                // make sure we subscribe to leave events for this window
+                trackLeave(hwnd);
+
                 QWindowSystemInterface::handleEnterEvent(window, localPos, globalPos);
                 m_currentWindow = window;
                 if (QWindowsWindow *wumPlatformWindow = QWindowsWindow::windowsWindowOf(target))