make seeking a lot smarter by invalidating the cache less frequently:
1. keep 1MB of old data around in case a backward seek is needed (playing
video files one get's to see a lot of those seeks)
2. make the buffer bigger: 1MB
3. when seeking 0.5MB behind the end of the current buffer, don't invalidate the buffer but just keep the datastream coming until the needed data is available -> no need to invalidate the rest of the data because it might very well be that xine wants to have some older data right afterwards