Improve wxBitmapButton best size for small bitmaps in wxOSX

The correct condition for adding the extra border to the buttons that
would be clipped without it due to the bezel style used for them is to
check that their height is < 20, as SetBezelStyleFromBorderFlags(),
which determines the bezel to use, does, and not if their width is 16,
as this is not the same thing, especially for non-square buttons.

Closes https://github.com/wxWidgets/wxWidgets/pull/1887

src/osx/bmpbuttn_osx.cpp

@@ -58,11 +58,17 @@ wxSize wxBitmapButton::DoGetBestSize() const
 
     if ( GetBitmapLabel().IsOk() )
     {
-        // Hack to stop 16x16 bitmap being clipped
-        if (GetBitmapLabel().GetScaledSize().x == 16)
+        const wxSize bitmapSize = GetBitmapLabel().GetScaledSize();
+        best += bitmapSize;
+
+        // The NSRoundedBezelStyle and NSTexturedRoundedBezelStyle used when
+        // the image is less than 20px tall have a small horizontal border,
+        // account for it here to prevent part of the image from being cut off.
+        //
+        // Note that the magic 20px comes from SetBezelStyleFromBorderFlags()
+        // defined in src/osx/cocoa/button.mm.
+        if ( bitmapSize.y < 20 )
             best += wxSize(4,0);
-            
-        best += GetBitmapLabel().GetScaledSize();
     }
 
     return best;