Bring OSX API in sync with MSW

include/wx/osx/bitmap.h

@@ -177,7 +177,7 @@ public:
     // these functions are internal and shouldn't be used, they risk to
     // disappear in the future
     bool HasAlpha() const;
-    void UseAlpha();
+    void UseAlpha(bool use = true);
 
     // returns the 'native' implementation, a GWorldPtr for the content and one for the mask
     WXHBITMAP GetHBITMAP( WXHBITMAP * mask = NULL ) const;

src/osx/core/bitmap.cpp

@@ -1867,9 +1867,9 @@ void wxBitmap::UngetRawData(wxPixelDataBase& WXUNUSED(dataBase))
     EndRawAccess() ;
 }
 
-void wxBitmap::UseAlpha()
+void wxBitmap::UseAlpha(bool use )
 {
     // remember that we are using alpha channel:
     // we'll need to create a proper mask in UngetRawData()
-    M_BITMAPDATA->UseAlpha( true );
+    M_BITMAPDATA->UseAlpha( use );
 }