Prevent negative time deltas.

As the comment says, I have several examples of Flashbang.update() reporting a negative time delta,
which understandably wreaks severe havoc, specifically in Flump Movie playback. Based on stack
traces I have, it appears that if you leave an iOS app running and let the iOS device idle, then
come back several hours later, getTimer() is probably confused, and you get several frames in a
row with bad values. I don't think it's an overflow in the math anywhere, I think it just takes a
couple of frames to get the app back into a stable state.