+ implement grain-merge blending mode (GIMP)

include/mapnik/image_compositing.hpp

@@ -68,7 +68,8 @@ enum composite_mode_e
     exclusion,
     contrast,
     invert,
-    invert_rgb
+    invert_rgb,
+    grain_merge
 };
 
 MAPNIK_DECL boost::optional<composite_mode_e> comp_op_from_string(std::string const& name);

src/cairo_renderer.cpp

@@ -382,6 +382,7 @@ public:
         case minus:
         case invert:
         case invert_rgb:
+        case grain_merge:
             break;
         }
     }

src/image_compositing.cpp

@@ -67,7 +67,9 @@ static const comp_op_lookup_type comp_lookup = boost::assign::list_of<comp_op_lo
     (exclusion,"exclusion")
     (contrast,"contrast")
     (invert,"invert")
-    (invert_rgb,"invert-rgb");
+    (invert_rgb,"invert-rgb")
+    (grain_merge,"grain-merge")
+    ;
 
 boost::optional<composite_mode_e> comp_op_from_string(std::string const& name)
 {