code: protected void tickAllMedia (long tickStamp)
      {
          // we use _tickpos so that it can be adjusted if media is added or removed during the tick
          // dispatch
          for (_tickpos = 0; _tickpos < _media.size(); _tickpos++) {
              tickMedia(_media.get(_tickpos), tickStamp);
          }
          _tickpos = -1;
      }
docs: Calls {@link AbstractMedia#tick} on all media to give them a chance to move about, change
  their look, generate dirty regions, and so forth.