form_builder issues

Believe it or not we are still using this module in some legacy code, and for the most it works quite well, however, I've just uncovered an issue:

When creating the subform in the element after build, we call: form_builder but form builder statically caches the current form that is being processed, meaning that the form that is cached is our subform. This is fine for the elements in the subform, but then subsequent elements will also get this form passed along, even if they are on the original form.

The only option I can see is to duplicate the form_builder function and allow the static cache to be destroyed.

Patch attached.