For Dataformats that support exporting HTML content, provide an API for
converting images within that content to something suitable for the format.
This fixes an issue with the PDF writer when it encountered a pluginfile.php
image, which it tried to request via HTTP without an active session. This
resulted in a 303 header returned by Moodle instead of the actual image,
causing an exception in the underlying TCPDF library.
Also removed write_header() and write_footer(). The reason for this
is because in core we want to know if the format being used supports
multiple sheets. To do this, and ensure we do not break third-party
dataformat plugins, we are using method_exist(). If write_header()
and write_footer() remain in the base class then this will always
be true.
