When there are a large number of sibling backdrop-filter primitives
present, the existing naive recursive traversals to assign the
`render_on` and `free_after` passes hit pathological cases. In
the test case for this bug, a debug build takes 12+ seconds to
complete on my development machine.
Instead, switch to a topological sort for `render_on` assignment,
and use a non-recursive traversal to set `free_after` (which relies
on having the passes in-order from the topo sort to avoid recursion).
Differential Revision: https://phabricator.services.mozilla.com/D144359
7004a2fbc3