The root cause of issue #8079 was a non-breaking space (U+0160).
`unicode.IsPrint` only allows the ASCII space (U+0020). Be more lenient
by using `unicode.IsGraphic` instead.
Fixes#8079
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#8057Fixes#4595
Updates #6731