Fix bug 6533: Spell-as-you-type stops checking words when they are already wrong

Fix bug 6533: Spell-as-you-type stops checking words when they are already wrong

The problem was in FontList::setMisspelled, where an optimization looked at the first character of the word in order to return early when the misspelled flag is already correct. This does not work when the first letters of the word have this flag but not the last one.

If this optimization was really useful, it can be reimplemented properly in terms of font spans.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@34765 a592a061-630c-0410-9148-cb99ea01b6c8