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/images/image.go

@@ -325,3 +325,9 @@ func IsOpaque(img image.Image) bool {
 
 	return false
 }
+
+// ImageSource identifies and decodes an image.
+type ImageSource interface {
+	DecodeImage() (image.Image, error)
+	Key() string
+}