cleanup of raw access to bitmaps:

1. remove UseAlpha() on platforms that don't need it and call it automatically from ~wxPixelData instead of requiring explicit call; deprecate wxPixelData::UseAlpha()
2. don't call UngetRawData() if GetRawData() failed

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@47295 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/gtk/bitmap.h

@@ -117,7 +117,6 @@ public:
     void UngetRawData(wxPixelDataBase& data);
 
     bool HasAlpha() const;
-    void UseAlpha();
 
 protected:
     bool CreateFromImage(const wxImage& image, int depth);