expose get/set for allow_overlap in MarkersSymbolizer

include/mapnik/markers_symbolizer.hpp

@@ -32,6 +32,15 @@ namespace mapnik {
     public:
       markers_symbolizer() 
          : allow_overlap_(false) {}
+
+    void set_allow_overlap(bool overlap)
+    {
+        allow_overlap_ = overlap;
+    }
+    float get_allow_overlap() const
+    {
+        return allow_overlap_;
+    }
     private:
       bool allow_overlap_;