improve WrongAngleFixer

- don't modify lines which are overlays for roads, no matter whether the
corresponding road was modified or not. This makes sure that overlay lines
are on top of or below the road, not next to it.

- use flag to mark overlay lines, this saves quite a lot of 
HashMap.containsKey() calls

git-svn-id: http://svn.mkgmap.org.uk/mkgmap/trunk@3248 25d90789-57f7-4ee0-8453-03a3dfeeeb22