Revert "revert the functionality of 944f34b - refs mapnik#2937"

This reverts commit a91d737.

Author: talaj

src/vertex_cache.cpp

@@ -155,14 +155,8 @@ vertex_cache & vertex_cache::get_offseted(double offset, double region_width)
 
     // find the point on the offset line closest to the current position,
     // which we'll use to make the offset line aligned to this one.
-    // TODO: `position_closest_to` is disable since it is too expensive:
-    // https://github.com/mapnik/mapnik/issues/2937
-    //double seek = offseted_line->position_closest_to(current_position_);
-    double seek = (position_ + region_width/2.0) * offseted_line->length() / length() - region_width/2.0;
-    if (seek < 0) seek = 0;
-    if (seek > offseted_line->length()) seek = offseted_line->length();
+    double seek = offseted_line->position_closest_to(current_position_);
     offseted_line->move(seek);
-
     return *offseted_line;
 }