Since spatial node indices are now stable across display lists,
we can include them in the interned data for clips.
This means that much of the code complexity related to clip
instances can be removed (though this is left for a follow
up patch once this has been in nightly for some time without
causing any regressions).
Long term, this means that clips are retained between display lists
along with their positioning information. We can take advantage of
this to cache state / dependencies about clips between different
display lists, which will eliminate some redundant clip work we
currently do every scene/frame build.
Differential Revision: https://phabricator.services.mozilla.com/D131962
998283bed3