Add support for clipping/cutting out (#435) (#448)

USAGE: overlayWith('overlayimage.png', { cutout: true } )

src/operations.h

@@ -14,6 +14,11 @@ namespace sharp {
    */
   VImage Composite(VImage src, VImage dst, const int gravity);
 
+  /*
+    Cutout src over dst with given gravity.
+   */
+  VImage Cutout(VImage src, VImage dst, const int gravity);
+
   /*
    * Stretch luminance to cover full dynamic range.
    */