added wxBitmap::UseAlpha()

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@20188 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/bitmap.h

@@ -146,8 +146,10 @@ public:
     bool operator==(const wxBitmap& bitmap) const { return m_refData == bitmap.m_refData; }
     bool operator!=(const wxBitmap& bitmap) const { return m_refData != bitmap.m_refData; }
 
-    // this function is internal and shouldn't be used, it risks to disappear
+    // these functions are internal and shouldn't be used, they risk to
+    // disappear in the future
     bool HasAlpha() const;
+    void UseAlpha();
 
 #if WXWIN_COMPATIBILITY_2_4
     // these functions do nothing and are only there for backwards