Switch SoundPlayerNode run mode to B_DROP_DATA when its latency
gets higher than buffer duration.

* The statements regarding buffer duration and latency from the
  earlier commit were correct, we just can't do it in
  BMediaEventLooper, every node has to implement this by itself.

* In a future MediaKit we should split up SetEventLatency() into
  SetInternalLatency() and SetDownstreamLatency() so it can be
  handled in BMediaEventLooper, making it easier to write correctly
  behaving nodes.