[FIX] web: more precise textarea autoresize

Setting 'overflow: hidden' on the (hidden) textarea used to compute
the (visible) textarea height ensures that there is no vertical
scrollbar, which would lead to a not so precise computation of the
height. The bug could be observed in sale.order form view, in the
order line x2many (description field), and had been introduced by
ef46378cd3.

addons/web/static/src/js/core/dom.js

@@ -114,6 +114,7 @@ var dom = {
             borderTopWidth: 0,
             borderBottomWidth: 0,
             padding: 0,
+            overflow: 'hidden',
         });
         $fixedTextarea.data("auto_resize", true);