Made callback on afterFrame be repeatable

summary: This allows people to return true from the afterFrame callback to sustain their function until the next time it occurs. 

reasoning: This would be very helpful certain situations, but would not break code in other situations. For example, I had player sprite and I wanted reloading to coincide with the animation, I could put reload() in the callback and return true if I was not down reloading. Instead of being able to do that though I had to throw an error instead to halt the flow so delete would not be called.