There is existing logic (see "commit_completion_waits_for_activation_") that blocks the main thread on commit until activation for WebGL and Canvas. To force double buffering, we can extend that logic so it blocks until we get a swap ack with the previous resources back. It would be relatively easy to implement, but would proactively block before the first non-deferrable draw command; if the app uses RAF like it should, we would block the main thread unnecessarily - but that might be okay for low end devices.

Alternatively, we could throttle the next BeginMainFrame until the SwapAck, which would free up the main thread and should work just as well for RAF based content.