pptx: Fix presentation rel numbering

Before now, the numbering of rIds was inconsistent when making the
presentation XML and when making the presentation relationships XML.

For the relationships, the slides were inserted into the rId order after
the first master, and everything else was moved up out of the way.
However, this change was then missed in the presentation XML, I think
because `envSlideOffset` was never set. The result was that any slide
masters after the first would have the wrong rIds in the presentation
XML, clashing with the slides, which would lead PowerPoint to view
produced files as corrupt. As well, other relationships (like embedded
fonts) would have their rId changed in the relationships XML but not in
the presentation XML.

This commit:

- Removes `envSlideOffset` in favour of directly passed function
  arguments
- Inserts the slides into the rId order after all masters rather than
  after the first
- Updates any other rIds in presentation.xml that need to be changed

0 files changed, 0 insertions, 0 deletions