Get notifications when pipelines are removed.

[nical]

We use epochs (see Renderer::flush_rendered_epochs) to track where the renderer is at and manage the lifetime of resources that are used by the renderer but owned by another thread. This works well except when we remove a pipeline, because we never get the notification for the last epoch of that pipeline and this results in leaks in gecko as we keep waiting for a rendered epoch notification that never comes.
Having special code to handle this in gecko is tricky because knowing that a pipeline is removed at the api level doesn't say anything about when it will not be used by the renderer anymore.

We need an equivalent of flush_rendered_epochs that signals all of the removed pipelines.

See https://bugzilla.mozilla.org/show_bug.cgi?id=1432708