Add ensureAlpha op, adds alpha channel if missing #1153

src/operations.h

@@ -30,6 +30,11 @@ namespace sharp {
   */
   VImage RemoveAlpha(VImage image);
 
+  /*
+    Ensures alpha channel, if missing.
+  */
+  VImage EnsureAlpha(VImage image);
+
   /*
     Alpha composite src over dst with given gravity.
     Assumes alpha channels are already premultiplied and will be unpremultiplied after.