Workaround to allow static methods for '#pre_render'.

For some components and configurators I plan to introduce in the future, I want to be able to use static methods in $element['#pre_render'].
Unfortunately this is not possible natively: drupal_render() calls function_exists(), which only allows procedural functions.

Let's introduce a trick for this:

$element['#pre_render'][] = '_renderkit_generic_pre_render';
$element['#renderkit_pre_render'][] = ['MyClass', 'myMethod'];