[filters] opacity() filter function?

I think it would be useful to add an opacity() filter function to the
Filters spec.  This isn't necessary for the 'filter' property (just
use the 'opacity' property instead), but would be nice when filtering
images via the filter() function (which appears to have not yet made
it into the draft).  Currently, the only way to make an image
partially-transparent is to use the cross-fade() function, which is
somewhat verbose and non-obvious:

cross-fade(image(transparent), url(foo.jpg), 20%)

I think the markup equivalent would be:

<filter id="opacity">
  <feComponentTransfer>
    <feFuncA type="table" tableValues="0 [amount]" />
  </feComponentTransfer>
</filter>

~TJ