Add support for negating only non-alpha channels

Fixes #1035

src/operations.h

@@ -45,6 +45,11 @@ namespace sharp {
    */
   VImage Gamma(VImage image, double const exponent);
 
+  /*
+   * Produce the "negative" of the image.
+   */
+  VImage Negate(VImage image, bool const negateAlpha);
+
   /*
    * Gaussian blur. Use sigma of -1.0 for fast blur.
    */