Prevent redundant, extra-long template suggestions

Problem/Motivation

I'm seeing extra-long template suggestions with repeated suggestions in the template name, e.g. input--submit--node-article-delete-form--op--node-article-delete-form--op.html.twig.

It looks like the pre-render function is being run more than once.

This happens when Drupal goes to render a form that has a #theme hook. I've confirmed this scenario with xdebug:

1. Drupal starts to render the form.
2. Drupal does the pre-render phase and calls Dazzler::preRenderForm().
3. Drupal sees the #theme hook for the form, it passes the $form as a variable to the form template.
4. The form template goes to render the form variable and re-starts Drupal rendering.
5. Bam. Drupal does the pre-render phase again and calls Dazzler::preRenderForm().

This is a core bug, but we can prevent this problem inside Dazzler::preRenderForm().

Steps to reproduce

The node delete form shows this problem.

Proposed resolution

Prevent pre-render function from altering the form multiple times.