images: Add images.Overlay filter

This allows for constructs ala:

```
{{ $overlay := $img.Filter (images.Overlay $logo 50 50 )}}
```
Or:

```
{{ $logoFilter := (images.Overlay $logo 50 50 ) }}
{{ $overlay := $img | images.Filter $logoFilter }}
```

Which will overlay the logo in the top left corner (x=50, y=50) of `$img`.

Fixes #8057
Fixes #4595
Updates #6731

resources/resource/resourcetypes.go

@@ -14,6 +14,8 @@
 package resource
 
 import (
+	"image"
+
 	"github.com/gohugoio/hugo/common/maps"
 	"github.com/gohugoio/hugo/langs"
 	"github.com/gohugoio/hugo/media"
@@ -59,6 +61,9 @@ type ImageOps interface {
 	Resize(spec string) (Image, error)
 	Filter(filters ...interface{}) (Image, error)
 	Exif() *exif.Exif
+
+	// Internal
+	DecodeImage() (image.Image, error)
 }
 
 type ResourceTypeProvider interface {