fix(tooltip): scrollbar flashing

When the tooltip is rendered for positioning scroll bars
may breifly appear depending on where the linked element
is positioned. This change adds a negative top and left
style to push the tooltip element out of view while it
is being measured and positioned.

Closes #4550
Closes #4623
Fixes #4458

src/tooltip/tooltip.js

@@ -123,7 +123,7 @@ angular.module('ui.bootstrap.tooltip', ['ui.bootstrap.position', 'ui.bootstrap.s
           'animation="animation" ' +
           'is-open="isOpen"' +
           'origin-scope="origScope" ' +
-          'style="visibility: hidden; display: block;"' +
+          'style="visibility: hidden; display: block; top: -9999px; left: -9999px;"' +
           '>' +
         '</div>';