trimmed formatter : wrong order of check_markup / trimming

Currently the 'trimmed' formatter applies a check_[plain|markup], _then_ calls node_teaser to shorten the text.
During a regular node view / preview, node.module does the opposite : generate the trimmed teaser (on submit time), and later run it through check_markup (on view time)

It's that order (trim then filter) that allows the 'html corrector' module filter to fix broken markup that occurs when the teaser gets cut in the middle of an opend tag.

Currently, html corrector filter has no action on our 'trimmed' text fields because they are trimmed after the filter is run.

Attached patch corrects this.