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

src/cocoa/bitmap.mm

@@ -483,10 +483,6 @@ void wxBitmap::UngetRawData(wxPixelDataBase& data)
 {   // TODO
 }
 
-void wxBitmap::UseAlpha()
-{   // TODO
-}
-
 // ========================================================================
 // wxMask
 // ========================================================================