Some Gecko display lists produce out-of-sequence ClipIds for ClipNodes
(#2898) which leads to empty ClipNodes. Previously the HitTester was
just discarding those nodes. This is an obvious problem because it means
that subsequent ClipNodeIndices are out-of-sync with the HitTester's
list of clipping nodes. This change fixes this by also adding empty
clipping nodes in the HitTester.

Unfortunately, we can't make a test for this until we figure out how and
why Gecko is producing display lists with out-of-sequence ClipIds.
Hopefully once we do that, we will no longer need to create empty
HitTester nodes at all.