Unbalanced tags across more and nextpage tags

[tellyworth]

It's easy to produce broken front page markup by including a --nextpage-- tag that breaks an enclosing bold or italic tag. There's some code in get_the_content that fixes this for --more-- tags, but it doesn't handle --nextpage--, and it'd be more efficient to do it at post save time.

The enclosed patch fixes this by splitting the content into slices at those boundaries and separately balancing each slice. Balancing happens in the content_save_pre action. No filtering is needed on the output side for posts saved after this filter.

It was a bit of a struggle figuring out where to fit this but I think the solution is fairly clean. It includes a new split_nextpage() function that can be used instead of ad-hoc regexps for splitting a post into pages.

By "No filtering is needed on the output side" I mean no _balance_ filtering of course.

Unit tests are in  http://svn.automattic.com/wordpress-tests/wp-testcase/test_post_filtering.php.

ah, yes, the number of sidebar gets knocked down issues that this might fix...

broken patch

see: #9765

fixed in r11398