Webform (render) element now uses $element['webform_build'] to store the rendered form

Previously when building a webform element, the rendered form was being merged with the $element's properties and causing unexpected results.

For example, the below webform element...

$element = [
   '#type' => 'webform',
   '#webform' => 'contact',
];

...would be processed and rendered as...

$element = [
   '#type' => 'webform',
   '#webform' => 'contact',
   /* rendered form */
];

Now, the processed $element will add the rendered form to the 'webform_build' property.

$element = [
   '#type' => 'webform',
   '#webform' => 'contact',
   'webform_build' => /* rendered form */
];