Login > Make background-cover work for IE8

The new background cover on login screen only work for IE9+
Here are 2 values to add for IE8:

filter: progid:DXImageTransform.Microsoft.AlphaImageLoader(src='.myBackground.jpg', sizingMethod='scale');
-ms-filter: "progid:DXImageTransform.Microsoft.AlphaImageLoader(src='myBackground.jpg', sizingMethod='scale')";

=> see how we can add this to Studio
=> add it by default in Nuxeo DM
=> check if it's enought since we force a compat mode, see NXP-12862.

If the user have the possibility to remove the behavior with Studio, this issue can be postponed.