Add contrast limiting adaptive histogram equalization (CLAHE) operator (#2726)

src/operations.cc

@@ -92,6 +92,13 @@ namespace sharp {
     return image;
   }
 
+  /*
+   * Contrast limiting adapative histogram equalization (CLAHE)
+   */
+  VImage Clahe(VImage image, int const width, int const height, int const maxSlope) {
+    return image.hist_local(width, height, VImage::option()->set("max_slope", maxSlope));
+  }
+
   /*
    * Gamma encoding/decoding
    */