This will allow experimenting with different representations of
the spatial tree (such as interning and/or providing stable
indices during display list building). It may also simplify
future changes to the public API to expose the spatial tree
directly.
As part of these changes, refactor how the debug representation
for the capture format is (de)serialized, to make it simpler to
add different payload vector types in future.
Differential Revision: https://phabricator.services.mozilla.com/D122183
db30aac4f1