Return smaller images for wxART_MENU/BUTTON under OS X.

Requesting images with client id of wxART_MENU/BUTTON used to return the large
32*32 icons because GetNativeSizeHint() wasn't implemented for these client
ids.

Moreover, under Mac some icons (notably message box ones) are created from the
corresponding icon bundle and the code in wxArtProvider::GetBitmap() didn't
resize them correctly in this case, fix this.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62299 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/osx/artmac.cpp

@@ -117,6 +117,14 @@ wxSize wxArtProvider::GetNativeSizeHint(const wxArtClient& client)
         // "32 x 32 pixels is the recommended size"
         return wxSize(32, 32);
     }
+    else if ( client == wxART_BUTTON || client == wxART_MENU )
+    {
+        // Mac UI doesn't use any images in neither buttons nor menus in
+        // general but the code using wxArtProvider can use wxART_BUTTON to
+        // find the icons of a roughly appropriate size for the buttons and
+        // 16x16 seems to be the best choice for this kind of use
+        return wxSize(16, 16);
+    }
 
     return wxDefaultSize;
 }