Scale maximum value in wxProgressDialog::SetMaximum() too

MSW-specific scaling was done for the initial maximum value in Create(),
but not if the maximum was changed later in SetMaximum(). Now do it
there as well.

Closes https://github.com/wxWidgets/wxWidgets/pull/1497

src/generic/progdlgg.cpp

@@ -600,6 +600,7 @@ void wxGenericProgressDialog::SetMaximum(int maximum)
     // we can't have values > 65,536 in the progress control under Windows, so
     // scale everything down
     m_factor = m_maximum / 65536 + 1;
+    m_maximum /= m_factor;
 #endif // __WXMSW__
 }