[PATCH] AFP PTX fields chaining

This patch fixes an issue in the AFP emitter, when a long text is split into multiple PTX fields inside a Presentation Text object.

The boundaries of the fields are not following the AFP specifications, leading to problems on some printers. It seems that the function chaining is implicitly broken at the chunk boundary and not at the end of the PTX segment.

The size of the PTX segment has been checked to respect the maximum length.
In the patch you will find a test case.