add markers placement methods vertex-first and vertex-last
visual tests - markers placement methods vertex-first and vertex-last
remove duplicate import
remove needless virtual inheritance
prefer c++ style comments
remove unused includes
Merge branch '3.x-szn-markers-symbolizer-vertex-placements' of git://github.com/mapycz/mapnik into mapycz-3.x-szn-markers-symbolizer-vertex-placements
Merge branch 'mapycz-3.x-szn-markers-symbolizer-vertex-placements'