Make the test for wxBUFFER_VIRTUAL_AREA in wxBufferedDC code more clear.

While "!a & b" is the same as "!(a & b)" when "b" is a single bit, the latter
is much more standard and more clear, so write this test (added in r71704)
like this.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74952 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/dcbufcmn.cpp

@@ -148,7 +148,7 @@ void wxBufferedDC::UnMask()
     int width = m_area.GetWidth(),
         height = m_area.GetHeight();
 
-    if (! m_style & wxBUFFER_VIRTUAL_AREA)
+    if (!(m_style & wxBUFFER_VIRTUAL_AREA))
     {
         int widthDC,
             heightDC;