fix(backdrop): use fixed position for global backdrops.

Set backdrop `position:fixed` when the parent is BODY to give it a position based off the body viewport.

Closes #2831.

Author: ThomasBurleson

src/components/backdrop/backdrop.js

@@ -48,6 +48,11 @@ angular
         // so let's wait until $animate is done...
         var parent = element.parent()[0];
         if (parent) {
+
+          if ( parent.nodeName == 'BODY' ) {
+            element.css({position : 'fixed'});
+          }
+
           var styles = $window.getComputedStyle(parent);
           if (styles.position == 'static') {
             // backdrop uses position:absolute and will not work properly with parent position:static (default)