Expose libvips warnings via NODE_DEBUG env var

lib/constructor.js

@@ -24,6 +24,9 @@ let versions = {
   } catch (err) {}
 })();
 
+// Use NODE_DEBUG=sharp to enable libvips warnings
+const debuglog = util.debuglog('sharp');
+
 /**
  * @class Sharp
  *
@@ -174,6 +177,8 @@ const Sharp = function (input, options) {
     tiffSquash: false,
     tileSize: 256,
     tileOverlap: 0,
+    // Function to notify of libvips warnings
+    debuglog: debuglog,
     // Function to notify of queue length changes
     queueListener: function (queueLength) {
       queue.emit('change', queueLength);