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/cocoa/bitmap.h

@@ -122,7 +122,6 @@ public:
     // raw bitmap access support functions
     void *GetRawData(wxPixelDataBase& data, int bpp);
     void UngetRawData(wxPixelDataBase& data);
-    void UseAlpha();
 
     wxPalette* GetPalette() const;
     void SetPalette(const wxPalette& palette);