Add CDN support for assets embedded in page wrapper templates

CDN support:

Code has been added to invoke CDN module's image src filter when rendering a page wrapper template. This ensures that any images embedded in the template have their src url updated to use CDN settings.

Input Filters:

check_markup() is now being called when the input type is not PHP. The Filtered HTML input filter strips php tags automatically via it's "limit available tags" filter. Since Full HTML does not have this by default,  additional code has been added to explicitly break php start and end tags if Full HTML is used.

I've tested this locally and all of the following are true:
* Images embedded in the page wrapper body are altered to use the CDN domain before the template file is written.
* When the input type is set to either Full or Filtered HTML, embedded php is disabled by breaking the php start and end tags. (<?php ?>)
* Region tokens ([content], [messages], etc) are added after content filtering is complete, in this way the php these tokens inject into the template is not filtered (props to Ben for spotting that).