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/gtk1/bitmap.cpp

@@ -1377,10 +1377,6 @@ bool wxBitmap::HasAlpha() const
     return false;
 }
 
-void wxBitmap::UseAlpha()
-{
-}
-
 //-----------------------------------------------------------------------------
 // wxBitmapHandler
 //-----------------------------------------------------------------------------