release update region as soon as we finished painting the window instead of keeping it indefinitely (bug 1819635) [backport from trunk]

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_2_8_BRANCH@49412 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/window.cpp

@@ -4528,6 +4528,10 @@ bool wxWindowMSW::HandlePaint()
     eventNc.SetEventObject(this);
     GetEventHandler()->ProcessEvent(eventNc);
 
+    // don't keep an HRGN we don't need any longer (GetUpdateRegion() can only
+    // be called from inside the event handlers called above)
+    m_updateRegion.Clear();
+
     return processed;
 }