9649 Roll buildroot to c5a493b25. (cla: yes)

9651 Move the mutators stack handling to preroll (cla: yes)

9652 Pipeline allows continuations that can produce to front (cla: yes)

9653 External view embedder can tell if embedded views have mutated (cla: yes)

9654 Begin separating macOS engine from view controller (cla: yes)

9655 Allow embedders to add callbacks for responses to platform messages from the framework. (cla: yes)

9660 ExternalViewEmbedder can CancelFrame after pre-roll (cla: yes)

9661 Raster now returns an enum rather than boolean (cla: yes)

9663 Mutators Stack refactoring (cla: yes)

9667 iOS platform view opacity (cla: yes)