#pre_render callback to append contents in #markup to #children.
This needs to be a #pre_render callback, because eventually assigned #theme_wrappers will expect the element's rendered content in #children. Note that if also a #theme is defined for the element, then the result of the theme callback will override #children.
$elements: A structured array using the #markup key.
The passed-in elements, but #markup appended to #children.
function drupal_pre_render_markup($elements) {
$elements['#children'] = $elements['#markup'];
return $elements;
}