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

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

src/common.cc

@@ -277,4 +277,17 @@ namespace sharp {
     return std::make_tuple(left, top);
   }
 
+  /*
+    Return the image alpha maximum. Useful for combining alpha bands. scRGB
+    images are 0 - 1 for image data, but the alpha is 0 - 255.
+  */
+  int MaximumImageAlpha(VipsInterpretation interpretation) {
+    if(interpretation == VIPS_INTERPRETATION_RGB16 ||
+        interpretation == VIPS_INTERPRETATION_GREY16) {
+      return (65535);
+    } else {
+      return (255);
+    }
+  }
+
 }  // namespace sharp