Update portal_transforms chains.

* update safe_html chain with new option default_encoding
* Add pdf_to_png chain to avoid conversion of pdf into transitional format pnmraw and
consume less memory.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@35624 20353a03-c40f-0410-a6d1-a30d3c3de9de