Fix visibility of crop mask with empty crop (#576)

Updates styling so that the crop mask (`.ReactCrop__crop-mask`) is
hidden, like the crop selection is, with an empty crop object (when the
`ReactCrop--invisible-crop` CSS class is active).

src/ReactCrop.scss

@@ -126,6 +126,7 @@ $mobile-media-query: '(pointer: coarse)' !default;
       outline-offset: -1px;
     }
   }
+  &--invisible-crop &__crop-mask,
   &--invisible-crop &__crop-selection {
     display: none;
   }