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.
* count() doesn't work quite as expected when the record being written
to the PDF is an object. So make sure to convert the record to an array.
* In addition, instead of comparing the total vs the current cell
counter, it would be more reliable to determine whether we're at the
last element of the array by getting the key for the last element and
comparing it with the key for the element that's currently being
processed.
Eloy Lafuente (stronk7)