Playback speed multiplier
Advances every registered root by the accumulated integer number of frames, then pull-syncs the whole tree to Pixi.
Driving is delegated to the original createjs walk: one _tick(evt) call
on the root advances the tree by exactly one frame (no delta is passed,
and framerate is fixed at -1, so advance() can never consume time
itself). A fresh event object is created per _tick call and shared by
the whole traversal, matching the granularity of the original
Stage-driven dispatch.
The original pipeline is TWO-phase: the tick phase only advances INDEPENDENT clips, and the draw phase (MovieClip.draw -> _updateState) resolves SYNCHED / SINGLE_FRAME / first-render state. There is no draw phase here, so updateStateForPixi() (the recursive _updateState walk) must run per frame as its substitute — without it, SYNCHED clips never move at all.
Whether to allow more than two frames to advance in a single process.