text simple placement: add center 'C' placement

include/mapnik/text/text_properties.hpp

@@ -96,7 +96,8 @@ enum directions_e : std::uint8_t
     SOUTHEAST,
     NORTHWEST,
     SOUTHWEST,
-    EXACT_POSITION
+    EXACT_POSITION,
+    CENTER
 };
 
 using fontset_map = std::map<std::string, font_set>;

src/text/placements/simple.cpp

@@ -57,6 +57,7 @@ struct direction_name : x3::symbols<directions_e>
             ("NW", NORTHWEST)
             ("SW", SOUTHWEST)
             ("X" , EXACT_POSITION)
+            ("C" , CENTER)
             ;
     }
 } names;

src/text/text_layout.cpp

@@ -79,6 +79,8 @@ pixel_position evaluate_displacement(double dx, double dy, directions_e dir)
         return pixel_position(-std::abs(dx),-std::abs(dy));
     case SOUTHWEST:
         return pixel_position(-std::abs(dx),std::abs(dy));
+    case CENTER:
+        return pixel_position(0, 0);
     default:
         return pixel_position(dx,dy);
     }