Implemented wxBitmap::HasAlpha() and ::UseAlpha().

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@32086 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/gtk1/bitmap.h

@@ -137,8 +137,8 @@ public:
     void *GetRawData(wxPixelDataBase& data, int bpp);
     void UngetRawData(wxPixelDataBase& data);
 
-    bool HasAlpha() const { return true; }
-    void UseAlpha() { }
+    bool HasAlpha() const;
+    void UseAlpha();
 
 protected:
     bool CreateFromXpm(const char **bits);