[A177r8] validate images with digest

[paulhiggs]

See A177r8 clause 7.5
when the @integrity attribute is provided with an image resource, validate the image against it. Currently only supports sha1 and sha256

[paulhiggs]

This update only checks the content digests provided in the @integrity attribute against the http/https loaded resource. Additional update is needed for inline images, i.e.

      <tva:MediaLocator>
        <tva:MediaUri contentType="image/png" integrity="sha1=:checksum:>data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAA...AAAABJRU5ErkJggg==</tva:MediaUri>
      </tva:MediaLocator>

Clause 7.5.3 of A177r8 says

If content digest is specified for an inline image it should only reflect the image payload and not the header.