This allows for constructs like:
```
{{ $filters := slice (images.GaussianBlur 8) (images.Grayscale) (images.Process "jpg q30 resize 200x") }}
{{ $img = $img | images.Filter $filters }}
```
Note that the `action` option in `images.Process` is optional (`resize` in the example above), so you can use the above to just set the target format, e.g.:
```
{{ $filters := slice (images.GaussianBlur 8) (images.Grayscale) (images.Process "jpg") }}
{{ $img = $img | images.Filter $filters }}
```
Fixes#8439
