Create Manhatten Intperolate method for -sparse-color on IM7 branch
diff --git a/MagickCore/distort.h b/MagickCore/distort.h
index fcb0c77..379b26a 100644
--- a/MagickCore/distort.h
+++ b/MagickCore/distort.h
@@ -66,7 +66,8 @@
     Methods unique to SparseColor().
   */
   VoronoiColorInterpolate = SentinelDistortion,
-  InverseColorInterpolate
+  InverseColorInterpolate,
+  ManhattanColorInterpolate
 } SparseColorMethod;
 
 extern MagickExport Image