Add support for negating only non-alpha channels

Fixes #1035

lib/constructor.js

@@ -179,6 +179,7 @@ const Sharp = function (input, options) {
     flatten: false,
     flattenBackground: [0, 0, 0],
     negate: false,
+    negateAlpha: true,
     medianSize: 0,
     blurSigma: 0,
     sharpenSigma: 0,

lib/operation.js

@@ -325,11 +325,19 @@ function gamma (gamma, gammaOut) {
 
 /**
  * Produce the "negative" of the image.
- * @param {Boolean} [negate=true]
+ * @param {Object} [options]
+ * @param {Boolean} [options.alpha=true] Whether or not to negate any alpha channel
  * @returns {Sharp}
  */
-function negate (negate) {
-  this.options.negate = is.bool(negate) ? negate : true;
+function negate (options) {
+  this.options.negate = is.bool(options) ? options : true;
+  if (is.plainObject(options) && 'alpha' in options) {
+    if (!is.bool(options.alpha)) {
+      throw is.invalidParameterError('alpha', 'should be boolean value', options.alpha);
+    } else {
+      this.options.negateAlpha = options.alpha;
+    }
+  }
   return this;
 }