[Roshambo] Sync stims with screen refresh rate

[mesca]

The current code does not make use of the scheduler.asap() method, which synchronizes events with screen refreshes. For an example on how to use this feature, see the oddball demo: https://github.com/timeflux/demos/blob/2c682f8d500be094855e3dee1400288f09ee703f/oddball/oddball/www/assets/js/app.js#L184