GUAC-1260 Need vendor extensions for background-size.

guacamole/src/main/webapp/app/login/styles/dialog.css

@@ -86,7 +86,10 @@
     margin: 0.5em auto;
     width: 3em;
     height: 3em;
-    background-size: 3em;
+    background-size:         3em 3em;
+    -moz-background-size:    3em 3em;
+    -webkit-background-size: 3em 3em;
+    -khtml-background-size:  3em 3em;
     background-image: url("images/guac-tricolor.png");
 }