but only the actual saving code, not the export magic and dialog.

Add new internal procedure file-pat-save-internal which is not
registered as a file procedure and always works non-interactively on
the passed arguments and only saves the passed drawable. Use the new
internal procedure from the file-pat-save code and remove all file
writing code from the plug-in.

This way all pattern file writing code duplication is killed, while
the whole export mechanism is completely unchanged.