add move constructor to vertex_cache

include/mapnik/vertex_cache.hpp

@@ -97,6 +97,7 @@ public:
     ///////////////////////////////////////////////////////////////////////
 
     template <typename T> vertex_cache(T &path);
+    vertex_cache(vertex_cache && rhs);
 
     double length() const { return current_subpath_->length; }
 

src/vertex_cache.cpp

@@ -28,6 +28,21 @@
 namespace mapnik
 {
 
+vertex_cache::vertex_cache(vertex_cache && rhs)
+    : current_position_(std::move(rhs.current_position_)),
+      segment_starting_point_(std::move(rhs.segment_starting_point_)),
+      subpaths_(std::move(rhs.subpaths_)),
+      position_in_segment_(std::move(rhs.position_in_segment_)),
+      angle_(std::move(rhs.angle_)),
+      angle_valid_(std::move(rhs.angle_valid_)),
+      offseted_lines_(std::move(rhs.offseted_lines_)),
+      position_(std::move(rhs.position_))
+{
+    // The C++11 standard doesn't guarantee iterators are valid when container is moved. 
+    // We can create them from indexes but we don't need to. Just let them uninitialized. 
+    initialized_ = false;
+}
+
 double vertex_cache::current_segment_angle()
 {
     return std::atan2(-(current_segment_->pos.y - segment_starting_point_.y),