Bugfix: IPD for footnote region now correct in multi-column layout.

Bugfix: IPD for footnote region now correct in multi-column layout.
Next step at multi-column layout:
- A multi-column section that needs column balancing is normally rendered until the next-to-last page. The rest of the element are re-broken by a special balancing page breaker.
- Multiple spans supported in area tree and through break handling.
- There are still problems with footnotes and column balancing.
- Main layout loop changed to render an element list right after it's broken. The block lists are not collected anymore and then rendered.
Bugfix: PageViewportProvider had a one-off (when accessing through "relative to current element list")

git-svn-id: https://svn.apache.org/repos/asf/xmlgraphics/fop/trunk@202368 13f79535-47bb-0310-9956-ffa450edef68