Add images.Opacity filter

[bep]

Fixes #11471

[jmooring]

This works great. At some point, processing without resizing will make it a bit easier.

This contrived example places a watermark over a base image, rotating it 90 degrees with 80% opacity.

{{ with $bi := resources.Get "images/base.jpg" }}
  {{ with resources.Get "images/overlay.jpg" }}
    {{ with .Resize (printf "%dx%d r90" .Height .Width) }}  <-- this is a hassle, have to reverse height & width
      {{ with .Filter images.Grayscale }}
        {{ with $bi.Filter (images.Overlay . 67 42) }}
          <img src="{{ .RelPermalink }}" width="{{ .Width }}" height="{{ .Height }}" alt="">
        {{ end }}
      {{ end }}
    {{ end }}
  {{ end }}
{{ end }}

[github-actions]

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