Fix wxOSX sizing of borderless wxBitmapButton

Don't apply the workaround required for NSRoundedBezelStyle and
NSTexturedRoundedBezelStyle, i.e. enlarging too small buttons, if these
bezel styles are not used. In particular, avoid enlarging buttons with
wxBORDER_NONE or wxBORDER_SIMPLE.

Follow-up to 0941b25.

src/osx/bmpbuttn_osx.cpp

@@ -66,9 +66,18 @@ wxSize wxBitmapButton::DoGetBestSize() const
         // 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);
+        // defined in src/osx/cocoa/button.mm and so do the style checks.
+        switch ( GetWindowStyle() & wxBORDER_MASK )
+        {
+            case wxBORDER_NONE:
+            case wxBORDER_SIMPLE:
+                break;
+
+            default:
+                if ( bitmapSize.y < 20 )
+                    best += wxSize(4,0);
+                break;
+        }
     }
 
     return best;