Fix some build warnings

private field 'm_dwCookie' is not used
'return' will never be executed
result of comparison of unsigned enum expression < 0 is always false
'FlushDC' overrides a member function but is not marked 'override'
potentially uninitialized local variable 'bound' used

src/msw/dc.cpp

@@ -1713,7 +1713,7 @@ void wxMSWDCImpl::SetLogicalFunction(wxRasterOperationMode function)
 
 void wxMSWDCImpl::SetRop(WXHDC dc)
 {
-    if ( !dc || m_logicalFunction < 0 )
+    if ( !dc )
         return;
 
     int rop;