Add prefix styles for legacy browser versions.

guacamole/src/main/webapp/styles/client.css

@@ -191,6 +191,9 @@ div.magnifier {
 .pan-overlay .indicator {
     position: fixed;
     background-size: 32px 32px;
+    -moz-background-size: 32px 32px;
+    -webkit-background-size: 32px 32px;
+    -khtml-background-size: 32px 32px;
     background-position: center;
     background-repeat: no-repeat;
     opacity: 0.8;
@@ -282,4 +285,4 @@ p.hint {
 
     box-shadow: 0.25em 0.25em 0.25em rgba(0, 0, 0, 0.75);
     
-}
+}