Ensure 16-bit input images embed onto alpha background

Support gamma correction of images with alpha channel
Favour shrink over affine when reducing by integral factor

src/operations.h

@@ -18,6 +18,11 @@ namespace sharp {
    */
   VImage Normalize(VImage image);
 
+  /*
+   * Gamma encoding/decoding
+   */
+  VImage Gamma(VImage image, double const exponent);
+
   /*
    * Gaussian blur. Use sigma of -1 for fast blur.
    */