[compositing-1] [compositing-2] Use bikeshed dfns for compositing modes

[Manishearth]

Currently all the compositing modes use manual <h4 id="porterduffcompositingoperators_foo"> + <a href=#porterduffcompositingoperators_foo> for cross-linking.

We should use Bikeshed <dfn> tags instead, which means that bikeshed-data can pick it up and other specs can easily normatively reference them and have the correct version (TR vs draft) appear in the "references" section.

[Manishearth]

cc @rcabanier

[dirkschulze]

@Manishearth The current editor is @chrishtr. Not sure if "bikeshedding" is on the priority list right now :)

[Manishearth]

Oh, sure, Rik's the one who added the references to the XR spec so I thought I'd notify him. It's not a big deal, and I'm planning on fixing this myself, just needed to be sure that this would be okay.

[AmeliaBR]

If you make a PR with the necessary markup fixes, it makes sense to get this into the Editor's Drafts, which should be enough to get auto-linking working from other specs. I'm not sure whether it would be worth a republication of the TR version, though.

[Manishearth]

Yeah, that makes sense.

[Manishearth]

#381