added InitAlpha() (replaces patch 991168)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@32213 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/image.h

@@ -272,6 +272,7 @@ public:
     unsigned char *GetAlpha() const;    // may return NULL!
     bool HasAlpha() const { return GetAlpha() != NULL; }
     void SetAlpha(unsigned char *alpha = NULL);
+    void InitAlpha();
 
     // Mask functions
     void SetMaskColour( unsigned char r, unsigned char g, unsigned char b );