Rename wxBitmap::GetImage() to OSXGetImage()

Make it more clear that this function is specific to Mac ports.

src/osx/core/bitmap.cpp

@@ -782,7 +782,7 @@ wxBitmapRefData::~wxBitmapRefData()
 
 bool wxBitmap::CopyFromIcon(const wxIcon& icon)
 {
-    return Create( icon.GetImage() );
+    return Create( icon.OSXGetImage() );
 }
 
 wxBitmap::wxBitmap(const char bits[], int the_width, int the_height, int no_bits)
@@ -949,7 +949,7 @@ bool wxBitmap::Create(CGContextRef bitmapcontext)
     return GetBitmapData()->IsOk() ;
 }
 
-WXImage wxBitmap::GetImage() const
+WXImage wxBitmap::OSXGetImage() const
 {
     return IsOk() ? GetBitmapData()->GetImage() : NULL;
 }