roof:shape=round added

![hbf2](https://f.cloud.github.com/assets/1747/286192/a8094250-9236-11e2-9f2a-562a41ab76bb.png)
http://city.informatik.uni-bremen.de/~jeff/hbf2.osm


The meshing should probably done manually (not with jts) but I couldnt figure out how your triangle strip code works.