Make wxOSX wxBitmap::CopyFromIcon() work as in the other ports

Just use the same object, as wxIcon is the same thing as wxBitmap
anyhow, instead of re-creating another wxBitmap which can be subtly
different from the original one.

Notably, converting an icon with default scale factor of 1 to bitmap
when using a high DPI display resulted in a bitmap with the same
physical size but scale factor of 2, as wxOSXGetImageScaleFactor()
returned 2 in this case, i.e. changed the logical bitmap size.

This couldn't be anything other than a bug, so fix this and, at the same
time, simplify the code and make it consistent with the other ports.

src/osx/core/bitmap.cpp

@@ -784,7 +784,8 @@ wxBitmapRefData::~wxBitmapRefData()
 
 bool wxBitmap::CopyFromIcon(const wxIcon& icon)
 {
-    return Create( icon.OSXGetImage() );
+    *this = icon;
+    return IsOk();
 }
 
 wxBitmap::wxBitmap(const char bits[], int the_width, int the_height, int no_bits)